Here's what you do
Write them a letter stating: "I forbid you to call me again." List your name, phone number, account ID (if applicable) and sign it. Then make yourself a copy and send them the origianl via registered mail, return recipet requested. If they phone again, you can take them to court and sue for damages for harassment. They can mail you all they want, but they aren't allowed to contact you via phone again.
I know this because I did it to student loan collector (on advice of an attorney of course) and walked away with a $900 judgement (they called me again after I sent the letter). I then called the original company that held the loan and used the $900 to pay towards my student loan
The amount you can recieve varies from state to state and also depends upon how much the lawyer is going to nail ya for ;)
Consult with an attorney though to see explicitly what your state laws are.