How do I evict a nightmare roommate in ca. he isn't on lease, & we have a verbal agreement. He has prepaid march ' & wants his money & sec. dep. Back now?

Why does he expect to get his March rent and security deposit back now? Has he said he will move out?

You basically have two options:

(1) Give him a written notice that his tenancy is terminated and he must move out within 30 days (60 days if he has lived there for more than 1 year). If he doesn't move out within that time then file a lawsuit for eviction.

(2) If he has agreed to move out immediately, I would give him back his March rent and security deposit on the day he hands you the keys. Even though you are entitled to 30 days notice (and rent) it would be worth it to give up that rent to get rid of the nightmare.