Collection Agency wont delete adverse info

ok so here is the deal, I rented a Apartment from a leasing company and the acting manager mutual agreed to terminate tenancy with NO amount owing, specifically as a result of several repeated maintenance requests, attempts to have essentials services such as heat restored. I effectively negotiated an amendment to my lease which was subsequently confirmed in writing by the apartments manager. Notwithstanding the above, my account was turned over to a Collection Agency and they have failed to remove the adverse info. I have submitted copies of the letter to both the CA and the CB but both has failed to remove, what are my options ?