How do you get a collection agency to stop calling you about an account that is not yours?

The Fair Debt Collection Practices Act 15 USC, Section 1692c(C) CEASING COMMUNICATION reads "If a consumer notifies a debt collector in writing that the consumer refuses to pay a debt or the consumer writes the debt collector to cease further communication with the consumer, the debt collector shall not communicate further with the consumer..."

So the magic phrase is "in writing". This type of letter is known as a "cease & desist" letter or C&D. Collection agencies only have to obey this provision of the law when it is in writing. Send your letter via certified mail, return receipt requested. If the collection agency contacts you again, you can sue them for violation of the law. You might also want to dispute this debt at the same time.