You have rights, and the right to peace of mind. Under federal law you have the right to sue for harrassment.
When you tell a debt collector not to call back and they do, they have broken the law. Get an attorney and sue. No cost to you. Google Bud Hibbs website and the ABC Night Line report titled "Debt Collectors Gone Wild" New laws have been enacted in response to Debt Collections Abuse.
Know your rights. Some debt collectors obey the laws and several these days clearly do not. If you are a debtor or a non debtor third party being threatened understand your rights and get a lawyer.
Rogue debt collectors count on you ignoring or not knowing your rights and what you can do. Before you initiate any legal action with an attorney it is wise to file complaints with your state attorney and the FTC online.