Harassment for a paid up account

I am being harrased by the collectors for an account that I paid up. I've send proof of payments & emails to Nimble the entire year, till today I have not received any feedback from them. When I asked Nimble to check on their system all the payments I made & share it with me, they couldn't. Another disturbing thing is that Nimble adds additional charges every month whether you pay your account or not.