I still think that mlp related content should stay in the thread. That seems like the simplest solution. Perhaps the first few times is a warning, the third results in a small ban and so forth.
i'm pretty sure this is how it's always been, because usually when it's posted outside the thread it isn't really relevant to the thread, and random irrelevance usually counts as spam which is, in turn, bannable
so, yeah. it doesn't really happen so much that it needs to be an actual rule, it's just common sense to ban the people who intentionally do this just to be a bunch of friends.