Being charged after cancellation

I cancelled my internet service on March 10, and called back 2 more times after that when i realized my service was not cancelled. on March 10th i was told i would get a $50 refund. I even got an email stating they cancelled the service. However i guess they didnt cancel it correctly, so a few days later i repeated the process and asked them to backdate the date to March 10th which they said they would do. Rinse and repeat on March 19th. Finally this one went through and I got a bill on March 19th saying It was cancelled correctly and I got a pro rated refund from March 19th forward, but would still owe them $100 for March 10th through the 19th. I tried calling them asking why i owed them money, considering i dropped off the equipment around the 12th, and they said on their end they showed i used their services up until the 18th, and even though their previous agents couldnt figure out how to correctly cancel an account, thats not their responsibility. that I NEEDED to take accountability for using their services (the phone supervisors words). I HAD ALREADY RETURNED THE EQUIPMENT BEFORE MARCH 19TH there was no possible way for me to be on that account. 

I tried to pay them and do a chargeback with my credit card so that it wont affect my credit score, and now i have an angry email saying if I dont pay within the next 10 days, they will send it to collections. I will likely end up paying the $100 and filing a BBB and FCC claim as they are literally stealing money from me, and what looks like hundreds of other people.