Debt collector harassment is defined as conduct that is harassment or oppresses the debtor. There are many examples of this under the Fair Debt Collection Practices Act, including:
If a debt collector does this, you can sue under the Fair Debt Collection Practices Act. Many debtors have successfully taken cases to court and won large verdicts against collection agencies and debt collectors who violated this law. If you choose to go this route, it would be a good idea to retain a licensed attorney in your area familiar with this area of law.
However, if you are simply annoyed with the phone calls and do not want them to continue, you can send a writing to the debt collector stating that they are no longer permitted to contact you on the debt. You should keep evidence of the mailing.