2 alts seems good. You still get some camping, but you just have to be willing to monitor it in SOME way and reach out. We do it on Arx, by counting poses done in a room with other characters. We don't have one set cutoff, we just kind of look at it like, if you had 0 poses the last two weeks, and only 20 the two weeks before that, I don't really call that very active on a roster character. Maybe make an OC, which has no activity requirement, and go from there. I suggest that, see if they want to talk about it, can be more active, etc.
I also think Apos is super right about the type of game environment you create (not shocking, I mean we obviously staff a game together). I think it's important for people to openly have the impression that being shitty, snarky, or mean towards other players is not ok, and I wade into many ooc channels to enforce that on a daily basis.
it's not fun. I get accused of attacking people, paying favorites, 'punishing' people, and trying to run a game WITHOUT FREE SPEECH, etc etc. You can't be conflict averse AND create the game environment you want, if you want any sort of standard of behaviour. You just can't, ESPECIALLY if you have some players in coveted positions/characters that others want.
Or you could maybe disallow all OOC communication, like an RPI?