:S I'd be freaking out.

Who owns the property? Is there a landlord? Do you own it?

Unless the basement guy owns the property... can you look into eviction?

If it's your property and you need the rent from him.... there's always people looking for a place to rent and once he's out and the place is cleaned up, you'll be able to find a new tennant.

I would be so stressed out in your situation. Like... if he was just a messy dirty hoarder that's one thing... but he's affecting the health of his cats, your cat, and you! Can you report him to the humane society or something for the neglect he is showing his cats?