How do I remove a credit card from passbook? Or set a password for it?

How did you add a credit card to Passbook? I didn't think it supported credit cards. Regardless, to delete a pass, tap the i button to flip the pass over and then tap the trash icon on the back of the pass.

You can't set a password for Passbook, but you can set a device passcode under Settings > General > Passcode.