He is probably new to it. This happens.
This is a core limitation of the medium
You are running into a core limitation of RP via text: you cannot police their writing as they are writing it and can only do so after the fact. Since they have the ability to resolve their own actions, and given they -- as the player, not the character -- are able to manipulate the world, their enthusiasm can lead to the situation you describe: what appears to be a godlike character, as the player is discovering the incredible amount of freedom this medium offers.
Give him time
With guidance and patience, this player will calm down. I know I was brimming with enthusiasm until about two months in when I first started. If he is new to this, then this might just be a "honeymoon phase" and it will pass.
Give him one power
There is a reason why the government legitimizes the lottery and casinos while holding gambling in general illegal: because it provides the would-be gamblers, who would gamble anyway and only try to find ways to skirt the law in this regard, a safer venue to play, and with government regulation. Otherwise, they would push gambling activities completely into the criminal territory and completely lose the ability to oversee and regulate it.
Learning from this wisdom, give your player's character one power, like X-Men. Let him know it's OK for him to do this one thing. This automatically carries the implication that other powers are not OK, and it constrains him in a way where he is still generally free. It legitimizes him playing god, but in a way you decide it's OK.
For example: he can turn invisible. Yes, but then what if he tries to push the boundaries? Can he make others invisible? Can he make things invisible? How many invisible things can he make and sustain at once? The great thing about this is it's all still the same trick (you can counter all of it with heatvision, radar, etc), but it encourages player creativity.
Lay down core rules that everyone can access
Games usually need a Session 0 to align everybody with what type of game you are all willing to play. Unfortunately, Session 0 doesn't work for your case. Instead, give everyone a link to a post with the core rules that everyone should follow. These are things like:
Once you have a "rule thread" then you can direct everyone to this, so everyone knows what is allowed and what is not. The rules should restrict the players' narrative freedom, but instead address meta concerns: things that, as players, they should or should not do.
Optional: Challenge him to improve his writing
The advice below is presented to you as a way of directly addressing the root of the problem (that I can perceive). However, doing the below is a lot of effort, requires a certain amount of group buy-in, and may be more trouble than it's worth.
Your player's issue is not simply that he is new, but that he doesn't get storytelling yet. Since he isn't intentionally trolling you, then this is the root cause of it. He has too much freedom and he is drowning in it, and he doesn't have a framework in which to move in. Help him understand storytelling by encouraging everybody to step up their game -- even you, as the GM. Sadly, only time and experience will truly solve this.
Elaboration: Challenge everyone to write up to a certain standard
Imagine if George R. R. Martin was one of your players. Would he create a god character? Or imagine J.K. Rowling, or Tolkien, or Suzanne Collins, or any other mainstream published author (too many to list!) playing in play-by-post together. Would any of them create god characters?
I can say confidently: no, they would not. This is because they are advanced writers and understand what it takes to write a good story. Your new player is not at that level where he understands this yet. If he did -- if his level of writing improved -- then he would not be writing godlike characters.
Consider a character literally dodging a bullet. This is pretty impossible for a human to do (almost as if the act was godlike). See how it runs if you do it plainly:
No writing knowledge
The mafioso shot him, but he saw it coming and dodged the bullet.
The above shows you a simple statement of what happened. Someone shot him and he dodged the bullet. In one sentence? That easily? He's superhuman!
Now watch what happens when you apply a bit of knowledge about writing elements into it:
The mafioso pulled out a gun and aimed it at him. Click went the gun's hammer as the bullet comes racing out of the chamber. There was almost no time to react. He closed his eyes and saw memories of his life. If there was only time to say goodbye.
And then he heard a ping! and opened his eyes. He was alive. Did the bullet miss him? No... he wasn't standing where he used to. He dodged!
In the above, you can see the dramatic structure applied. Exposition, rising action, climax, falling action, resolution, the whole deal. And it doesn't read like a god character dodging a bullet anymore. In fact, it sets up a mystery (how did he dodge?) which is actually more thrilling to read that the first one.
Let us crank that quality a little higher.
The mafioso shot him. In that instant, he saw his life before his eyes. The warmth of his wife's kiss, the smile on his son's face, the laughter of his jovial father turning to tears. Was this it for him? Had he fucked up so badly that he had to die for it?
When he heard the bullet's lound ping!, he wasn't standing where he used to. He had dodged the bullet.
A bit more advanced that the Beginner one, but the differences are subtle. One of the big changes is summarization: the first paragraph was summarized into one sentence, but retaining the same impact. Meanwhile, we insert character motivation into the scene, which allows us to empathize with the character. Finally, a shift in voice (notice the curse words?) as well as change in pacing, which signals that we are not in the third-person omniscient perspective anymore, but in the third-person limited, and it shows us who the character is. Keep in mind it still follows the dramatic structure.
Now, I hesitate to show an "advanced sample" without editing it for days... but I've been training myself to write since I was nine, and at some point in my life, I think I did reach that level. But despite the fact I haven't written anything for a few years now, if for nothing else, here is an "advanced" sample of the same scene which demonstrates the benefits of improving your quality of writing, to address your issue of a player creating a god:
The mafioso shot him. In that moment, he was dead. He felt pain in his chest. He felt his wife's kisses turning cold, he heard his father's laughter turn to crying, he saw his son's smile fading into darkness.
For a moment, there was silence. And then, the gunman broke it: "What... it can't be! You... you're not human!"
He was breathing heavily, on all fours, a cold sweat beading down his forehead. He had moved out of the bullet's way. Grimacing, he said: "It's going to take more than that to kill me, bitch."
What has improved: Writer voice, organization of sentences, dialogue, nowhere are thought verbs used (as suggested in a very good essay by Chuck Palahniuk, author of Fight Club), thoughtful use of the word "said," etc etc.
Now imagine your play-by-post only having "advanced" levels of writing. (It doesn't have to be, but gods will it be awesome.) You have no god characters, but rather protagonists who live tension-filled and exciting lives. There is always conflict and action.