From the Editors of The Daily Cat
Some cats, like humans, exhibit a tendency to dominate. If your male cat appears to have claimed the litter box as his own, this behavior may be right to some extent if you haven’t put out enough litter boxes for your cats. The situation you describe is like a single stall for a bunch of concertgoers. Everyone has to wait in line, crossing their legs, while the stall claimer goes.
If you have at least three cats, you should make four litter boxes available to them. The general rule is to provide a litter box for each cat, plus one. So if you have four cats, you should put out five litter boxes, and so on.
The Humane Society of the United States further recommends that you place one litter box on every level of your home. Your cats would otherwise have to travel quite a ways to go. The separation might also help your pets to better define territory. However, sometimes cats do copy each other as a sort of cat version of schooling behavior. In that case, the boxes would be placed in the same general area. Remember that a clean box does not smell, so any number of boxes can be kept together so long as you properly maintain them.
Should one of your pets eliminate in an undesired spot, the Humane Society recommends covering those places with upside-down carpet runner or aluminum foil, citrus-scented balls, or water bowls, since cats don’t like to eliminate where they eat or drink.
You may not be able to retrain your male cat out of his dominant ways, but you can make life easier for your other cats by providing a sufficient number of litter boxes for them.