In California, Occupants in possession amazingly have more rights than unpaying tenants. The process for an unlawful possession is hugely expensive as well. Worse, in California, there are flocks of people who are professional theives who prey on you "rich" owners. They know the system and can work it as well as the best attorneys. If you have this problem do seek a legal consultaion but before taking action buy them out of the property. Make them a cash offer that you will pay on the day the property is vacated and returned to your possession.