My uncle is 80, is showing signs of Alzheimer's and let some young couples live in his spare room. Is there any way we can legally force them out so caregivers can move in?

They are now considered tenants...by law.
Your Dad will have to evict them.

This is a length process and not going to happen quickly.

I suggest...your Dad just move. Give the landlord notice..and go. It will be lots easier for the landlord to get rid of them based on a "lease hold-over" than for your Dad to now make the claim of non-payment of rent..or anything else...since he allowed them there for no rent to start with.

Failing all that. Dad needs to get a lawyer to handle this..and quickly.

If it is an apartment, talk to the apartment manager about getting them out. If there are more than one couple, it probably violates the lease agreement for the apartment. It may mean that your uncle will need to move. This doesn't sound like a bad idea for him, really, if he is renting.

With neighborhood friends like that, who needs enemies??