NOT CANCELLING MY CONTRACT

i sold my vehicle and need my tracker cancelled. i have phoned three times to cancel my contract. Call centre tells me some one will phone me back in 72 hours. No-one phones so i phone them again. Explain to the second guy and he assures me some one will phone me that same day. Yet again no phone calls its been 3 days and still no calls. PLEASE CALL ME TO CANCEL MY CONTRACT OR I WILL GET MY BANK TO STOP THE DEBIT ORDER. HOW CAN I PAY FOR SOMETHING I AM NOT USING? WORST SERVICE I HAVE EVER HAD IN MY LIFE WILL NEVER USE TRACKER AGAIN.