How can I rid myself of micro billing software, demanding bill payment?

i stupidly downloaded free trial website subscription withouting noticing terms and conditions, stating that free trial only lasts for 3 days. consequently I am receiving micro billing sytem deliveries now demanding payment, I have tried to uninstall software but it just keeps reappearing, threatening to bring my computer to a standstill..clever stuff!!! I have cancelled the subscription. Should I just pay up the demand and request that they arrange the unistallation..or is there another way ?