I am hesitant to answer this as purely "Talk to your players," but this will come close and it is the almost necessarily first step.
- Talk to your disruptive player.
It's important to figure out why your player is doing this, and unless they are actively malicious at a game-sabotaging level, talking to them is probably the best way to find out. Be polite, be respectful, be willing to share why you think this is a problem, but find out why they do this.
At this stage, it's also worth just asking them to stop, or at least dial it back. If they comply, problem solved!
- Talk to your non-disruptive players.
It's also important to figure out why they tolerate or go along with this behavior, and maybe even more importantly just how hacked off they are. If you are misjudging the situation and the other players are good with this, you have one situation. If there is a rift-- half the group is indulgent (possibly for social reasons) and half the group isn't, you have a much different and potentially more volatile situation.
- Stop stimulating and/or rewarding the player.
Once you understand why they're doing this (if they persist) you can figure out how to reduce or remove the rewards they're getting.
- If necessary, remove the player.
We hope it doesn't get this far, of course.
That framework is, to be honest, common sense. So much so that it's almost useless. I've had more than one player like this, though, so I'll follow up with a few examples:
- Plot-Hook Boy
I had a player once who sounds very similar to yours-- always going off on weird tangents (action tangents) based on the slimmest of evidence. I wasn't smart enough to just ask him why at the time, but in casual conversation about gaming in general he once volunteered that that was just his play style. I still remember something close to direct quote: "I'm gonna follow up on whatever the GM puts out there-- if there's a dog by the side of the road I'm gonna play with that dog." (It was almost a rant. It went on in that vein for the length of a paragraph.) Everything was a plot hook for this guy. It also turned out he had the expectation that every rumor his character heard in a bar was meant to be followed up on, whereas I was spewing out a bunch of true and false ones as background flavor. If I had described something rotting in a forest, he would have done something similar to this, although he probably could have been restrained by the other players.
I ended up having to dial back a little on my descriptions and to be more explicit than I prefer about what is flavor text and what is not. Today, with more experience, I would probably resort to telling him that his character knowledge tells him this is not a big deal. (I.e., imply he made a knowledge check.)
That guy was easily bored, so it wasn't a perfect solution, but it worked enough.
- Acting Out Guy
Another notable example was a player who was fighting the structure of the whole campaign from day one. It was a relatively simple prophecy-driven game and any time the prophecy came up, this guy found something else to do.
This time I was smart enough to just ask why, and his reason was, he didn't like that style of game and really wanted to play in a sandbox game. "Prophecies are dumb," featured in his answer. So essentially he was trying to make the game into a sandbox game. (As it happens, my tastes have evolved now to be similar to his then, but that's not the point.)
The other players were there for the discussion, and they were fine with the premise of the campaign. After a few more sessions it became clear that no one was willing to change so we... basically stopped notifying him of the next game date and he basically stopped showing up. (This was quite graceless of us. We were all young.)
Back to the Framework
It's unlikely that you'll have something that exactly matches my examples, but hopefully the basic framework will help. In other situations I would consider things like:
- Let the character go his or her own way, but keep the spotlight time divided appropriately (i.e., do not reward them with excess spotlight time, if that is their reward.)
- If necessary, structure things with a deadline, to encourage the player to stay on track, but also to encourage the other players not to follow them. (If jerking the other players around is their reward.)
- The world is a dangerous place. Let there be warnings, let there be consequences, and let them fall where they may.