martin. Get a water pistol and "shoot" the offending cat when it misbehaves! They soon learn what you don't want them to do!
Originally posted by SpencerTheLion:
...what I'm about to try is proping the foam-reinforced posterboards between the litterbox & the litter pan (it's in a corner).

This might be a bit risky, but I'm also going to spray an ammonia cleaner on the walls, so the cats think it's 'marked'. Don't want to go overboard and discourage them from the litterbox!