You don't have squatters you have tenants who do not respect the lease and provisions within it.
I think the easiest and most cost effective way to deal with such a matter is to not renew the lease when the time comes. If you are on a month to month basis then give them a 30 days notice when it is appropriate.
If you have a term lease you know the end date and confirm with them with ample time that they will be moving as the lease states at the end of the lease.
Next time screen your tenants better. Find someone who has ample income to rent your property so they will not want to have other unpaying people living with them. Make sure their credit score is such that if they do the wrong thing they are going to lose something they have worked for.