She is your subtenant. You have to evict her. Serve her with a 30 day notice of termination of tenancy. At end of 30 days if she is still there, file an eviction lawsuit.
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Legally speaking - You can start with calling the police, who may tell her to get out since she's not on the lease. There's not enough information on your post to tell whether or not she's just a visitor/guest or a subtenant - but in either case, write her a letter and tell her that she has breached your agreement about staying in your house, and you demand that she leave or you will sue to evict her and seek all of the costs and expense in doing so from her. If that doesn't work - you probably need to hire a lawyer to help with this process.
Practically speaking - is this the mother of your children and are they there, too? Any chance she will claim you are currently common-law married? Are you ready to sue her to get her out?