RP cannot be enforced.
Not really.
Not without turning the world into a prison camp stalked by style police.
If someonei is disruptively OOC, then explain to them what is wrong with it.
If they do not respond, or become OOCly abusive, hit /report <abuser's name>, /tell a GM, and ask for help.
Sometimes that does not work, but usually it does.
If the OOC behavior is not disruptive, consider it background noise, like a loud cicada on a hot night, and ignore it.
It is not worth the trouble.
Most importantly, do as much RP in /say, in front of others, as possible.
To increase the amount and quality of RP, you must be shining examples, that others will emulate.
Making RP look like more fun that what the OOC players are doing, will have a greater effect than any rule possibly could.