Get competent legal advice before doing anything.
Be extremely cautious of following Greg's advice. In most states & commonwealths you must post permanent signs that not only state "No Trespassing" but you must also have the code section on that same sign. Then it is an enforceable misdemeanor crime. Otherwise it is not enforceable most places.
here is an explanation:
The thing I would like to point out is IF you were to tell the tenant to get rid of them as Greg suggests, you might be violating her rights. What if a tenant has someone visiting for the holidays, are those people also trespassing? and will you do the same thing to them?
Now the tenant that has the problem son & guest, you could possibly refuse to renew because those guests are nuisances, but most likely you will have to notify her in writing.
there are two things I would strongly recommend you use:
1.. a book called "neighbor laws" by a competent lawyer- available for purchase & download
2.. a website called understanding contract law and you win-