At the top right of the iTunes store click "Account" under Quick Links. Sign in and then next to payment information click "Edit". You can then select none as a payment method.
Also see here:
Welcome to the Apple community.
If your account is being compromised, the best course of action is to change your password.
Start here, change your country if necessary and go to manage your account > Password and Security.
If you are unable to remove your current payment information, you might try putting a block on payments to the iTunes Store through your card provider.
I'm not sure they can help, but if you contact Apple, they may be able to remove the information for you.
Express Lane (select your country, navigate to iCloud help and enter the serial number of one of your devices)