Ideally, try to locate the letter. Under the Fair Credit Reporting Act, you can formally dispute any thing on your credit report with any of the credit bureaus and/or the creditor directly. They then have to respond.
The problem for you is that you probably do not have a copy of the handwritten agreement. Thus, it is going to be their word against yours. If you have another person who witnessed the transaction, that would help you. You could keep fighting until they file suit, then try to reach a settlement. Depending on the amount owed, you probably should go speak with an attorney.
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If you have a copy of the handwritten agreement signed by you, ex-boyfriend, and apartment manager or representative, write a letter to all three credit bureaus denying responsibility for the debt on the lease agreement. If you don't have a copy, did your ex-boyfriend have one? Try to get one if he kept a copy. Consult with an attorney with experience in consumer, debt collection, and debt reporting laws for direction and action.
My response herein is an attempt to give you general information and direction and is not intended to constitute an attorney-client relationship as perceived by state law.