Tracker charges for cancelled contract

I've been a tracker customer since March 2013 till November 2018. I cancelled my account with tracker and was told I had to give a months notice (like most service providers request) I understood and waited for the final debit order, it came and another debit order after the 'last one'. So I reversed the money since I had served my notice period, now I'm receiving calls from tracker's very rude debt collectors saying my account is in arreas and I'm now owing 3 times more than the reversed amount. I didn't know tracker charges intrest on cancelled and services not rendered.