Sadly this community is infected with an epidemic of cross-gaming encompassing incidents of revenge, favors, friendships, points, etc. It's sickening (pun intended), and even more disgusting that some of the "better' players here engage in it. It's extremely easy to spot, but nearly impossible to prove. The only way to stop it is to stop tolerating it as a community. As kaner406 has mentioned the moderators try to track this stuff, but it's very difficult because evidence of it doesn't necessarily occur through actual conversations on the site (yes, the moderators can read all of your messages). The reality is that while it's against the rules, there's next to nothing that the moderators can do to stop it unless they unearth overt evidence of quid pro quo behavior occurring across games.
Metagaming is the wrong word for it, because there are applications where metagaming is perfectly valid (tournaments, series of games, and SRGs). But Cross-gaming is just out and out cheating, and it destroys the integrity of the games and the community.
The answer to this is to build a community where players enforce this rule against cross-gaming thorough mutual respect of each other, and condemnation of the behavior when it goes on. There's no way to effectively penalize it officially, but if enough people cared about it passionately, the behavior would stop because you reputation in the community would suffer if you're observed doing it.
Ideally we could all just play on our honor and not do this. But they are many here who do not respect the game, and do not respect the rest of us as fellow players enough to give the common courtesy of playing each game with a clean slate, free of any off board influence.
I say that the problem is at an epidemic level when otherwise respected players here engage in this behavior routinely, and it goes unmentioned and unsanctioned. Everyone can always hide behind the' it's not allowed to discuss cheating on the forums' clause to shield them, thus killing any public observation of it. And then the only remedy is to approach the moderators, can only prove the sloppiest and most overt cases of cheating. I understand the reasons for keeping cheating accusations out of the public forum, but there's got to be a better solution than sweeping it under the carpet and saying it doesn't exist. It does exist, and it's destroying the community. But then again, there I go again assuming that anyone actually cares if this place has any respect for itself.