Melissa,
Have you approached your Guest about this problem? Assuming you have not broken any of your house rules or airbnb's, I would do several things right away: Talk to your Guest, hopefully he/she can try to rectify the problem, be firm and let them know that it must be taken care of immediately. Remind them that they have violated airbnb's TOS. Send your Guest an email through the airbnb email platform so that you and airbnb have a record taking extreme care to be factual, and well balanced. Call airbnb, while doing all your coresponding through the airbnb email platform and lastly after you called airbnb and discussed the situation with your Guest if they have not rectified the situation ask them to vacate verbally and through airbnb email. Do Not Cancel! Let airbnb do the heavy lifting. Hopefully they have not been there longer than 30 days as your options and recourse diminish dramatically. The most important thing to do is; document, document, document, and don't break your own house rules or airbnb's.
Good luck and keep the Community Help posted. Be tough, you can do it.
Eloise