Never give permission to bank accounts or post dated checks with these companies. Always send a money order or cashier's check. Never talk to them on the phone. If you answer, politely ask them to send you something in the mail, you never know whos on the other end of the line. Also, you can send them a first class letter, return reciept requested asking them to stop calling you at home, work, etc, calling your neighbors and such. They are legally entitled to do so, unless you specifically tell them no. If they continue, you can counter sue.
I'm learning this the hard way.