Harassment from Nimble Group

My contract with ADT was terminated in 2016 due to a poor service and I was on a month to month contract and In August 2016 after paying my last payment I informed them I was canceling my contract,furthermore after trying to retain my account I still maintained I wanted to terminate. The contract was then terminated without any further deductions and services.

I am now shocked to get call from a tracing agency called nimblegroup that i owe ADT some money after terminating the contract in 2016.The people from Nimble group have been harrasing me since March 2018 even though i informed that i dont owe them anything and sent numerous emails to them confirming that but nothing changed.They went as far as promising me a 20% discount if i pay.