If a debt collector is constantly calling you, one of the easiest things to do, particularly if it's a cell phone, is tell them not to call. It's called revocation of consent and I'm referring to the TCPA, the Telephone Consumer Protection Act. This law is very useful. It provides 500 to 1500 per damage for every phone call after you've told them not to call.
It's important to write down the date and time, maybe this person that you spoke with as well. If they tell you that it's still legal for them to call, they're actually incorrect. Under the TCPA, here in the state of Florida, there is case law that says verbal revocation of consent to call is perfectly legal and is enforceable against them. You can simply tell them not to call. Keep track of all future calls and then consult with an attorney about your rights.