Anyone who has a cat or had a cat, please help answer my question!?

we have two persian cats, (brother and sister adn they are both fixed) Cayman, and his sister Meeka. They were born in July 2010, so they are of decent age. however, my dad is threatening to do something about them if we don't fix their problem. They are litter box trained, and use the litter box(especially Meeka. SHe's never gone to teh bathroom on the floor unlike her brother) but they get it attatched to them or something and they get it stuck to them and then it's all over the floor. Yesterday it was all over teh hallway and the wall and my dad was so upset he was in a horrible mood and threatened to "do something" about them. I'm really worried for them, because they have become a part of our family, adn I love them, but i don't know wha tI can do about it. Cayman is worse than meeka, but they are both horrible with it. They are very affectionate, and great cats, but they are very messy. Meeka at least licks herself, but Cayman we have to give a bath because he doesn't clean himself. They are very dirty cats. Is there anything we can do to help them? We've tried everything. We've tried even locking them in the room that the litterbox is in, adn then just leaving them in there overnight, and nothing seems to work!!!! Please help us!