Q: how to remove a credit card

Just remove it, which device are you using.

I believe that if you have family sharing set up you must maintain a payment method on file. You should be able to change to a different credit/debit card if required, but not switch to none.

tt2

Change or remove your Apple ID payment information - Apple Support

iPhone 4s

how do I remove my payment information

Log into your account, not when trying to download something, and see if you get the 'none' option - click on the link in itunes999's post above for instructions.