If someone has been there a day or two, maybe...but for several MONTHS....not even close. Police are going to tell you that this is a civil matter and refer you to your local civil court.
If they are not living somewhere else simultaneously, if they are getting mail there, or have a key, or are keeping their stuff there and sleeping there on a regular basis (or any number of possible "tests" for determining), then they have established residency. Regardless of a lease, or payment of rent or utilities or any other bills, once you let someone establish residency, they are your tenant under the law. You'll need to follow the eviction process to get them out if they won't go willingly. You can find info and the necessary forms online at:
http://www.courts.state.co.us/chs/co...s/fed/fed.html
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While pointers can be helpful, ultimately the number one lesson in any legal action is: don't take legal advice from books, family, friends, co-workers, police officers, grocery clerks, web sites, or people on legal message boards. The only person who can give YOU legal advice is YOUR attorney.
http://www.aardvarc.org