Any online shoppers?

Does anyone use any apps on their phone for online shopping? If so, do you know how to take your credit card off or block the person that you bought from? Because I think someone from an online app that I used has my credit card number and is taking money from me somehow... I checked my account balance and the numbers just don't add up right. It said the transactions were made from that app and I know I haven't bought anything else from them.