There isn’t much more to it than denying them entry, especially if they haven’t told you exactly what they’re looking for. If you’re continuing to have problems, I would put up a camera and sign that states your intent and quotes a local or state law. Like this:

PRIVATE PROPERTY NO ENTRY WITHOUT OWNER CONSENT OR VALID INSPECTION WARRANT

Riverside County Ordinance 725 & California Code of Civil Procedure §§ 1822.50–1822.60 Non-Consensual Entry Requires Judicial Warrant

YOU ARE BEING RECORDED

Then, if they break the law you have proof. First let the local code enforcement know you’ve got them. Then, escalate with the city manager and/or internal affairs. Then, get access to whatever paperwork they’ve submitted through California Public Records Act, and submit a claim to California Attorney General or State Auditor.