I remember reading a story in the recent Priceonomics book about a similar tenant conflict in SF. Tl;dr, a 75-yr-old lady was thrown out into the cold, but only after months of hearings and lots of money wasted on lawyers. Why do such laws exist at all in CA? Why not just send out several warnings in advance, explaining whole situation and the available options for tenant?

These two guys are just asking for a good old ass whooping. Throw them out, then let them try and prove that they were renting the place to begin with. Whatever the premises, you could certainly make it be your words against theirs.