When you do a purchase, there is a tick box on the payment page that says
"remember my payment information for next time".
If you don't want your payment information saved, you have to leave this box unticked.
Leaving the box unticked will mean that any future purchases will require you to enter your payment information again.
If you are concerned that a hijacker can get into your account and use your credit card then you need to leave that box unticked.
If you are concerned that a hijacker can get into your account, maybe you need to review your email, device and account security.
There is a very good set of instructions to work through on this page:
Security tips