You may be able to get some free advice from the Austin Tenant's Council http://(www.housing-rights.org). One of their services is telephone counseling for Travis County residents. Out-of-town callers will be assisted if resources are available.
You should be able to rekey your locks if you give the landlord new keys (check your lease). My favorite locksmith is The Lock Doc (512-444-4333).
By the way, most leases allow guests to stay for only a few days a month. Allowing a guest to stay longer than allowed by the lease could put you in jeopardy of eviction.
None of the above should be considered legal advice.
Keller Williams Realty
Be careful who you allow to stay in your house....you're giving up your rights if you don't have a written agreement. Even if you have a written agreement, eviction is unpleasant and you still have to go through the process.....I know I'm a landlord for my own properties. Good luck.
Best of luck to you!