I don't know exactly where to draw the line either. Too tough you're going to discourage the regulars from maybe making an attempt to be funny or make some levity, and either way newcomers, plain old shitposters will just do so because such is in their nature.
The only real suggestion I can make would be if some /kc/ splinterchan goes down maybe make like 7ch and shut down for a couple of days in order to head them off. It may confuse people that don't check in everyday and would require the mods and admins to be on top of such places at the sake of their own sanity but it would also prevent newcomers from finding the site as easily, good or bad.
There is no easy solution. It seems like every imageboard, subsection or subculture gets overrun eventually.