Nimble group is *****ulent and a ****. They **** people

My MTN account was handed to debt collection company and i paid it off and sent the paid up letter to MTN in 2020. After that in 2022 Nimble group started calling me about the very account that was paid off and closed. I sent them the paid up letter but they ignored it and now the account appears on my credit report. They have done this before and they keep appearing whenever they feel like it. They do not care about the paid up letter they just want me to keep paying for an account that was paid off. I believe that what they are doing is *****ulent and they are taking advantage of citizens. They have seen the paid up letter so what else am I supposed to pay for? This **** cannot go on, this company should be held accountable for not conducting their practices with integrity