They could add a global game RP chat group, anyone can post in it, but if some number of players flag person as out of game chatting, he gets a 5 min not able to post in that channel penalty,.
So it would be like main chat, but community would decide what is RP in game discussion and what should be in non RP chat.
some algo with decreasing increasing time penalties could be implemented also.
Although info messages at penalty would have to be friendly to keep new people from getting upset or feeling like it is a cliche