Threatening sms

I had a Alltech netstar installed in a vechicle which I didnt realize I had to cancel once I sold the car, once Creditworx phoned me and told me I was going to be handed over ,as I had not cancelled the tracker, I immediatedly contacted Alltech netstar cancelled it, and paid creditworx. A few months later Creditworx contacts me again and says I still owe money, after many phonecalls and emails I gave in and just paid the amount out of fear for a blemish on my credit record. However this time I did ask for a letter stating that the amount paid is now full and final payment. So I suppose not much of a surprise I get another sms today !!!! Payment required TODAY... BAD DEBT, I really just dont know what to do anymore.I have spoken to so many people both at alltech and creditworx.