Clients who are actively enrolled into a membership are able to bypass any prepayment rules you have set in place during online booking. But, the membership must be ACTIVE for the client to bypass prepayment.
If a client's membership is Paused, Pending, Cancelled, or Expired they will no longer be able to bypass prepayment rules. So, while a client with a membership in one of these states may still have available membership credits, if you require all clients to prepay during online booking, the client will be required to submit a prepayment to book their appointment.
The only workaround to this would be to book the member's appointments for them until their available credits have either expired or been redeemed.