I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today. I have been a CA licensed attorney for more than 3 decades, and I handle real estate law and landlord-tenant issues.
This seems like a very crucial matter for you, and your questions and issues suggest that an in-depth conversation might best suit your needs. If you are interested, for a very nominal charge of just $5 more I can offer you a personal phone conference during which there is no limitation on the number of questions you can ask of me, as opposed to continuing in this question and answer thread which is searchable and viewable by the public.
Let me know if you don't want a call and I will be happy continue here with an answer for you.
Either way, whether I call you or not, in order to give you a clear and concise answer, I will need some additional information about the circumstances, please.
1. I presume that both these people are in your home---your primary residence?
2. Do you have a traditional rental agreement?
3. What grounds under your rental agreement are you allowed to evict?
4. What did the letter that you placed under his door say, specifically?
Thanks in advance,