I would go ahead with the request for debt validation, and in the same letter, advise them that they are to cease and desist collection efforts by phone or mail until they can verify the debt is actually yours. Also let them know that you will not communicate in any other form other than by mail, in writing. Send your letter via certified mail, return receipt requested, so that you have proof they received it.
I wouldn't be worried about a lawsuit - if, as you say, the account is not yours, then a lawsuit would go nowhere!