Threatening SMS's

I have had a Netstar membership when I bought a new car, it was included with a 1 year subscription. After one year I did not want to continue. I got a statement that said I am in arrears, I cancelled the subscription and paid the outstamding amount due. Until this day they keep on sending me threatening messages that I still owe them an amount of R330, which I paid and send them the POP. They don't have the desensy to phone me or reply on my e-mails. You just can't get hold of them.