The Fair Debt Collection Practices Act gives you the right to stop collection calls. All you have to do is send the debt collector a letter telling them to cease all communications. There is no magic language required--just say in plain-English that you don't want them to contact you anymore. Keep a copy of the letter for your records and mail it certified with a return receipt so you can prove that the collector received it. Once the collector receives your letter, they must stop contacting you immediately. There are a few caveats here, though:
If you want to resolve the debt, you may not want to stop all calls. If your goal is to work something out with the debt collector, you probably want to keep the lines of communication open. But if you can't afford to pay or if the debt collector harasses you, it may make sense to cease all future calls.
The cease request will not stop any legal action. The FDCPA only requires that the debt collector cease communications with you. Courts have found that a person's cease request does not require a collector to stop or forego any collection lawsuits or garnishments.
The cease request only applies to the collector you send it to. Many debt collectors will respond to a cease letter by transferring the account to a different debt collector. In most cases, your cease request to the first debt collector won't apply to a subsequent debt collector. You will likely have to send that subsequent collector another cease letter.
Debt collection scammers may not honor your letter. Unfortunately, there are a few debt collection scammers out there. These fraudsters are just trying to get your money--there may not even be valid debt. Compliance with the FDCPA is not a high priority for a scammer, so they probably won't honor your cease request. The best thing to do with these scammers is to make it clear to them that you refuse to pay.
If a debt collector receives your cease letter and continues to contact you, they've probably violated the FDCPA. One of the most important rights the FDCPA gives a person is the right to stop collection calls. If a collector violates this right, they should be held accountable for their illegal conduct. Consider talking to a lawyer in your area who sues debt collectors under the FDCPA.