Staff not cancelled contract as requested

I Have had a tracker in my vehicle since 2007 and have religiously paid all this time. Because my vehicle is old and not a high risk I no longer require the tracker. I phoned and cancelled my contract and also email to confirm as I was contacted by Tracker. It seems that nobody understands 'I HAVE CANCELLED THE SERVICE'. I keep getting smse's for payment and on Friday received an sms advising that my account has been handed over. This is absolutely not acceptable. I am a pensioner and have a good track record and I take exception to this treatment. I would like to email or phone the CEO or COO as it would seem that the staff have not carried out my instruction to cancel the contract. I also don't believe an sms is the correct way of communication with a client and is highly unprofessional. Surely a person can contact you telephonically or by email and you can deal with the correct person. I did contact the collection department on Friday but they were unhelpful when I advised that the contract was cancelled and that I don't owe Tracker anything. This matter better be resolved immediately.