Send them a letter asking / telling them to cease all communication with you. After that, they can send you one last letter. If they continue to communicate with you after that, they are in violation of the Fair Debt Collection Practices Act and you have grounds to sue them.
I agree, write them a letter, send it certified return receipt, and tell them that the statute of limitations (4 years) has expired on the debt, the 7 years that it may appear on your credit report has expired and that you no longer want them to communicate with you. Contact an attorney to draft the letter if you need help.
If they are attempting to collect on a debt when they have no legal right to do so you can sue them under the Fair Debt Collection Practices Act. However, keep in mind there are some things that may affect the statute of limitations. Contact a local FDCPA attorney.