When is it justifiable to cut someone out of your life?

So my mother's boyfriend has a daughter who lives with us. She has for about a year now. For the. past year she has done nothing but lie, steal from us, sold our stuff, done drugs, and basically disrespect us in any way possible. Recently She has told a close friend of mine a very personal secret while they were arguing. She wasn't even mad at me, but she still brought it up for whatever reason. I've tried to be nice, friendly, and forgiving, but honestly that was my last straw. I don't want to be around her any longer. I have tried to mend things before, but I don't even want to at this point. I guess what I'm asking is, what do I do? I can't live with someone and ignore their existence, but I also cannot keep pretending not to despise her. I am also a very calm and forgiving person by nature, and I guess I'd feel guilty if I cut her off. Any advice? Or just use the poll.
Cut her off.
Vote A
Keep trying.
Vote B
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When is it justifiable to cut someone out of your life?
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