Cancelled my contract because of bad service but they are still billing me

After I cancelled my contract with them In January because of bad service they sent me a cancellation invoice of which I paid the amount. However it seems they actually continued to bill me and they keep on calling me. I give them all the evidence of the settlement invoice and the amount paid. They keep on saying the account was not closed and you should continue paying. Even after giving them the evidence that I closed off the account and telling them that if you didn't close it please close the account, yet they still send me bills and have sent me emails that I have been handed over.

Never use this service for tracking. When you actually need them to help you they don't, but when it comes to money they go full on. Very bad company. I say it again never use this company.