Collectors calling for the wrong person by: DebtCollectionAnswers.com
Are you able to get any kind of contact information for this debt collector - their name and address? If so, you can send them a certified letter telling him that they have the wrong number and insisting they do not call you again. You can send a copy of that letter to the Federal Trade Commission and your state attorney general.
You can also contact a consumer law attorney for help. There have been some cases where consumers successfully sued debt collectors who harassed them for a debt that did not belong to them.