Nimble trying to collect on paid up accounts

Nimble tried collecting on two accounts that have been settled long time ago. I don't owe money on these accounts. They added it to my credit report which is affecting my credit score. I have told them numerous times i do not owe any money on these two accounts i have paid up letter and a letter from them that they are ceasing collections due to account paid up. Three years along the line here they are sending me a whatsup collecting on both accounts again. Is it so difficult to see the proof close the accounts and take it off my credit record for once and for all? I am being pushed to the amount where i am going to involve legal aid and file a case of ***** against Nimble