Try changing to an uncovered litter box, cats will harass each other in the covered ones, they wait outside the box and ambush the one using the litter box, . I have seen my cats do it.
If all else fails get the largest crate you can find and confine the offenders (one cat per crate) in there with a litter box bed food and water for a week as a last resort. also go to http://www.zeroodorstore.com/?OVRAW=...OVMTC=standard and buy a bottle of this stuff, i use it and it works unlike natures miracle and some of the other stuff. Also get some cat attract litter and mix it with the regular stuff.