I try to keep personal OOC to myself, and if OOC overtakes the RP, I sometimes suggest a group for OOC discussion. Obviously I slip up sometimes, but then, I'm human (even if my characters aren't)
I said yes as my answer, not because I have anything against grinders and so-called PLers, but because I believe that RP should be more prevalent. Leveling and RPing can coexist. Right now it can be a little hard because grinding takes up a lot of time, but even still, I think everyone should be expected to remain in character, whether they choose to RP or not. I also think maybe an 'OOC' channel may be an idea? With an option to show it in the 'Main' tab. However, brackets should still be used just in case. But this would rid people of OOC in RPs if they wished, and Non-RPers could live in peace without RPers getting annoyed at them for being OOC. There could be a setting in the ingame chat options to choose the channel your normal text shows in, with 'OOC' or the 'Main' channel. This way, Non-RPers could easily use that as their 'Main' while RPers have 'Main' nice and clean, and can use the 'OOC' channel just like anyone else.
If that were implemented, I think regulating the OOC at GM events and such would be easier, and RPers would be happier, and Non-RPers would be left in relative peace.
There would of course be some room for error for new players who wouldn't necessarily know of this. Perhaps, rather than suggesting roleplay in the tutorial, saying that staying in character is a RULE, and that OOC chat must go into the OOC channel is a RULE.
I'm not certain how hard/easy any of that would be to create, but maybe as a future idea...
Enforcing roleplay directly, like, telling a non-RPer he can't play because he is not a roleplayer, would be rather rude and a little to far. After all, roleplay is not the ONLY purpose of the game, it is just highly encouraged, and I believe it should remain that way. What should be focused on is the encouragement of RP. There are many people that would be willing to RP if it was explained to them more thoroughly. I doubt that most people actually read that part of the tutorial (I didn't, really, when the tutorial was first made). So perhaps more of something you have to read before the tutorial or after it, and you have to wait a certain amount of time AND scroll to the bottom to go through. Make the words somewhat large, people would be more likely to read it (unlike EULAs when installing software, which most people never read).
EDIT: Wow, I think that's one of my longest posts on these kinds of threads yet >.>