You have proof that the debt is listed in error? Hang onto it, as you will need it.
Experian are dicks, in my experience. Send them a letter demanding removal of this item immediately. State in the letter that they have thirty days to comply, otherwise you will sue them silly for violating the Fair Credit Reporting Act. Give them thirty days, because they update on a thirty day cycle. Enclose a copy of your proof. Send the whole shebang to them certified mail, return receipt requested, and keep the return receipt card to bolster your case.
If they have not complied within thirty days of receiving your letter, file suit. It's an easy thousand bucks, and they're begging you to take it.