Don't use the monsters as depicted in the Monster Manual.
If the players constantly look up the stats for monsters... alter them. That way they can't use the MM and if they argue that it's not in the book like that, tell them, that that's not just legal, it's actually encouraged by the DMG, MM and other books. We had discussed that part of DMming in the question Is it wrong to use monsters other than how they appear in the Monster Manual?
The GM is always right!
As a subpart of that discussion, you can point to that loaded sentence. Did you know that The GM is Alway Right was coined back in the days when the first edition of DND was published? Well, in any case, it is the beat all argument, that is still true in D&D 5e, and the only justification you need to alter statistics of beasts.
Make the table expectations clear
When I run a game, one of the first things I do is establish sonme kind of table expectations. Besides an introduction to the safety-tools I use (Timeout T as a gesture to halt a scene that's uncomfortable as an X-card variant, Lines and Veils, Stars and Wishes), I also form expectations about player behavior. Typical for a game where the bestiary is a huge part of gaming, one part of the expectations would be a statement akin to this:
Please don't look up the adventure or monsters you face. You'll ruin your own fun. I do modify the possible beings you face to better fit the situation. I might reskin monsters to better fit the place. If you actually need to look up a beast, do so quietly and without disturbing play. And please, if you do, don't tell the others.
Tell them to stop the annoying behavior
As a basic thing, telling a player that something is annoying should be expected. Unless you voice concearns, you can't come to a compromise. However, in this situation it's not just annoying to you, it is actively hindering you in running the game, and that makes it a behavior that needs to stop. The best is to be clear about this. It doesn't matter if it is excessive description of violence, looking up the adventure or monster stats: if the behavior annoys you or the other players, tell them to stop.
"Do you want to run the game?"
A GM I like to play with has a very humorous way to adress rules-lawyers and too nosey players: he asks one question.
Do you want to run the game? Here, I make room, take over right here and now...
In my circles, I have always seen that a player scolded this way stands down from whatever they were doing to annoy the GM with - be it rules lawyering or looking up enemy statistics. Surprisingly, I have yet to encounter - and hear of - a single case where the player accepts and not stand down.
Repeat misbehavior might warrent removal
If a player continues to be obnoxious and annoy everyone, you are not facing a good player, you are facing a problem-player that actively makes the game unfun for you or other players, then there is one last step to consider: letting the player go and telling them not to return is an option.