Can I turn off credit card fee for specific clients?

Yes, it is possible to turn off the credit card fee for selective clients. You can do that by creating an additional ePayments profile under Settings > ePayments and turning off the processing fee for that profile. Then, go to the client record (Clients screen) and change its default ePayments profile to the newly created no-fee profile. You can use this same no-fee profile for other clients too. Check CORE Help center for details.