I got cheated...

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I was looking for ways to start for myself a bussines online and bumped into this company called ONLINE SUPLIERS who were then supposed to supply me with goods for the customers I would find on E-bay. On registering I gave them my credit card details and they took $50 from my account even though they had promised me 15 days trial without any charges. I requested for a refund through email but no one replies my mails. I now only want to cancel the service but don't know how. I am afraid they will steal my money again next month. I can't close the account because there are more important monthly charges to be taken from it that I can not change at the moment. Has anyone dealt with these guys before? How can I protect myself from further thefts.