The strongest squatter protection is thorough screening before anyone ever gets the keys. Verify credit. Confirm employment and income. Look at savings and assets, and actually call the references. A tenant with stable finances and something to lose is far less likely to stop paying and dig in, so most of your real risk gets managed at the application stage, long before move-in. Be honest with yourself about the limits, though. Screening lowers the odds without eliminating them, and once someone is lawfully in the property, removing them runs through a legal process that takes time. What a landlord actually controls is the quality of who gets in, so put the effort there. California's landlord-tenant and unlawful-detainer rules are state-specific and they change, so confirm the current process and your obligations with a local attorney or property manager before you need them. Good screening plus knowing the correct legal steps in advance is the best realistic defense you can build.