If its invite only you should just tell the "rules" and tell him there are younger member in that room and ask him/her about his/her age and if he's okay with explict rp stuff. if the person isn't okay with it don't invite? Not sure if this is too Obvious or if im just being stupid and not reading the post good enough.
Eitherway people who do this stuff even though they know the rules shouldnt be in the room.
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Edit: Yupp, me being stupid
Give him a warning I guess? or kick him for a little while just make sure he doesn't do it again.