If you are getting constant calls from a collection agency demanding money, there is a few easy ways you can stop the calls. One way is to write the collection agency and tell them to stop calling you. They have to comply per the Fair Debt Collection Practices Act. It is advisable to send your correspondence certified, return receipt requested. This way you have a record that you sent the letter to them. If they continue to call after they receive your letter, you can sue them in court.
Here is another way you can stop the collection calls. It is unorthodox, but it actually does work! Unlike the previous method listed, it will not cost you anything! Check out this video for instructions on how to proceed: