People follow to the Clearing all the time - some characters just like to see fights. And, some characters still want to help (or hinder), IC, regardless of whether their help is wished. Yeah, others may think that character rude or at the very least, too nosey for their own good; but, following into the Clearing is something that can have a lot of in character reasons - the Clearing is right there, next to the tavern, anyone can wander in.
Now, following into Carpe DM or Ahoy, Adventure! is a bit different. Those rooms are settings that are determined by those within in them when they're being used. It is, as the subject line of this thread says, a point of etiquette to not do so. The setting could even be a completely different world - so, how would the other characters get there? Etc . . . So, if someone comes in uninvited while you're working on a plotline that others would not know of, politely pm - no need for name calling, anger or annoyance. There's the possibility that it's a person who doesn't read the boards, or is new to ISRP, so doesn't know the purpose of those rooms (though, Carpe DM does hold a hint in its very name; Ahoy! Adventure may sound like a sea faring room or something if someone doesn't know any better). So, patience. Try pming first. If it's truely disruptive and a major issue, and the person clearly knows what they're doing, maybe the Magi need to be involved.
Do we have a place where the intentions of those two rooms is posted clearly? I remember the naming threads, and a discussion of whether or not a DM would be required, but, what's the full purpose? And what came of the discussion of DM requirement? Maybe a description of intent as you enter, the way the other rooms have physical descriptions would help?
I like having the free setting rooms, and I imagine it must be annoying when people just show up and spy or interfere when they've no in character reason for them being there (I haven't seen that happen, personally - I'm not denying that it does, it just hasn't happened when any of my chars have been there). We can't make being polite a rule, though, it's too ambiguous, and, besides, some rude characters can be quite amusing. I think it's best just to do as this thread is doing - raise awareness.