I will assume that your cats are grown and that you are using the typical clay litter. This being the case, your cats could be eating litter because they are bored, or because they are sick, or because there is something lacking in their diet. The first thing you must do is take the kitties to the doctor. The kitty doctor can make sure that nothing is wrong with your kitties, and she will even be able to suggest solutions to your problem. If your cats have a clean bill of health, then you could try using a different brand of litter.
Some cats have strange tastes, and like to lick or eat certain chemicals.
Maybe there is something added to the litter that tastes good to your cats. Another brand wouldn't have that chemical added and wouldn't taste so good to them. Make sure your kitties get an approved diet and plenty of exercise. A good diet is important to make sure that they get all the minerals they need. Exercise is to keep them from getting bored.