The best thing to do.....immediately get in contact with At&t and let them know your situation and establish a passcode on your wireless account. Also you may want to reset your password for your att.com username and password.
The best thing is to be productive if you have someone interferring with your service. I assure you once you metion this to At&t they will reach a resolution for you the same day!
Hopefully this assists you :)
