How do I get them to stop calling me about someone else?

I keep getting calls from a company called Cavalry Portfolio Services. The calls started a couple of months ago. The keep asking for a person by the name of MXXXXX J. MXXXXX jr. (I'm not going to give his name). Problem is I don't know who this person is. When I answer they ask for him by name, and I tell them they have the wrong number. Now they have left several messages that state If I'm not MXXXXX J. MXXXXX jr. I should delete the message and not listen to it. (yea right). They go on the inform in the message that they are attempting to collect a debt .....give a reference number etc. I have even called them direct and told them I don't know who they are looking for and they keep telling me they will remove the number, but they don't.

Seems to me they are breaking the law, by leaving messages that someone else owes money on my answer machine...

Please, how can I get them to stop? I"ve filed a complaint with the FTC, but the calls keep comming. They call on adverage about 2 time a day. I've had the same phone/address since 1999, and I honestly don't know who they are looking for.