Legal Question in Real Estate Law in California

You have to serve them with appropriate notice to quit, and then file an unlawful detainer lawsuit.

You have to evict, but in that eviction suit you get money damages in addition to an order for eviction.

Persons whose initial possession of property was lawful, like tenants on expired leases or former owners, aren't trespassers. This is an unlawful detainer and your remedy is eviction, as the previous answers say.