Honestly, I feel like lots of players don't know what meta-gaming is. It's a more complex 'don't' rule than godmoding, because it's hard to know whether you're doing it or not; it's much easier to tell if you're controlling somebody's character than making sure you're working with the correct body of knowledge (your character's, rather than your own). Honestly, if somebody meta-games in your RP, make sure they understand what meta-gaming actually is, and then ask them to not do it. Sometimes they'll be fine with it; othertimes, they'll try to justify their bullshit.

I was once in a game on another forum with a player who, with no means of telling and no reason to suspect, instantly wheeled on my character, who was secretly withholding information from the group, to ask if there might be anything he wasn't sharing. This was based on my character's private thoughts from my post and no actual clues. When I pointed this out, I was told that their character was "very complex". There was nothing that I could really do, certainly not from a player's perspective, to stop their nonsense, so in the end I just made my excuses and invited somebody to kill my character off.

As a GM, my general rule is that I look for a minimum writing standard (spelling, grammar, originality) but a slightly higher 'attitude standard'. If I think somebody is going to be bullish, uncompromising, incorrigible or inactive, I don't accept them into my RP's. Obviously this is difficult to gage, but if you can do a bit of research on your players, you can see how they write and also how they respond to constructive criticism. Also stick 'no meta-gaming' in any 'rules' section your game might have, so that, if you see it, you can refer to the rules and not have to argue about whether it was right or wrong; it was wrong, because it says not to do it in the rules.

As for the second situation, there really isn't anything you can do about that other than vetting every idea your players come up with, and you won't get very far like that!

[quote=@Ellri]
Our favorite when we have doubts about a player and/or character?

Run their character through the [url=http://www.springhole.net/writing/marysue.htm]Mary Sue Test[/url]. If it goes above 30 or so, its not good. Optimally, it should be well below that too.[/quote]

It doesn't help with assessing a player's writing ability; only whether their character is clichéd or not, surely?