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While a house guest is not technically a tenant, certain tenant-landlord laws still apply to the relationship if they've been with your for more than 30 days. Talk to an attorney who will help you draft and send an eviction notice. Giving this advance warning, in writing, is essential to protect your liability. This warning will legally establish them as an "at-will tenant." You need this status if you have to pursue legal action, so do not skip it. Be wary how you word the letter so that they can't use tenant laws to prevent you from evicting them. Check your individual state's policies, and make it clear what type of living arrangement you had with the person, especially if they are not paying any rent. If they still won't leave, you can take them to court. If they paid for groceries or any bills, they may legally be an "at-will tenant," making it much harder to kick them out legally. If they ignore the first written warning, you'll need to file a formal eviction proceedings with your local district court in order to get them out. Generally, your letter will outline a place for them to receive their belongings in the event that they don't move, as well as the specific date their stuff will be removed from your house. If you suddenly lock out an at-will tenant, especially if their belongings are still in the house, you could be the target of costly civil suits and legal action. Changing the locks on a guest, if it causes problems or cuts them off from their property, can even get you jail time in the wrong circumstances. Moreover, it often inflames already high tensions and can lead to further issues. Once you have a court order, and/or have notified the police that you are worried about your safety, you can safely change your locks. Unless they are a legitimate resident of the house, usually determined if they receive mail or are on the lease, they can be removed from your property as a "trespasser." Obviously, involving the police is for the most extreme cases, and even the mentioning of 911 is often enough to finally get someone out the door. Some police offices will refuse to get involved in a matter like this. However, if you've sent the letter and/or filed for eviction with a court, they will come remove your guest as a trespasser.
Send a certified letter asking them to leave in 30 days or less. File an official tenant eviction order with your local courts. Do not change the locks unless you are worried about your safety. Call the police if they still refuse to leave.