Non-refundable option on reservation

Hello Sir/Madam

I don't want my clients to have a refundable option when making an appointment.When they make an appointment, I just want the non-refundable option to appear. Although I tried many times on the Booking extranet,It didn't work.May I please learn about how to do that?

I am looking forward to hearing you soon.

Best Regards.

Tayfun Evren