I would start now with some changes.

Put their non-perishable foods in a box in their bedroom. Allot a small section of the fridge for their use. Put your cats bed back in its place. And schedule TV time.

Is there a TV in the guest room?

Do these step by step so that they get the idea they need to leave by the end of the month.

And post a few proverbs around the house and include the one above.

And bless you for putting up with that. I'm allergic to tobacco smoke, so any friend of mine would have to live in a pup tent out back.