The next time they call, politely explain that you don't know the person, politely request that they stop calling, politely verify the phone number (if you are returning the call from a message), if you are getting mail politely explain that too.
Remember, debtors are rude to debt collectors and hang up on them. The more you do that the more it seems you are the actual debtor and are just dodging. If you are polite and the calls don't stop, politely ask for a supervisor or manager, they will help you stop the calls.
Sending a nastygram in writing will only enforce the belief you are the debtor. It may stop the calls, but when the debt is sold or reassigned they will just start again from a new debt colelctor. Sending a polite letter will help break the cycle.
Finally, if you get the calls spaced out by weeks and/or months, they are likely skip-tracing the debtor and finding your information over and over. You probably are connected to the debtor some way, via address or somesuch. Be polite and they will do their best not to waste time calling you. Debt collectors want to collect money, not harass innocents, so if you help them understand that you aren't the debtor, they won't WANT to call you.