ian the idea of pre reg is to allow you to input rider info onto the system before the race to save time.
riders still have to come to registration, sign a slip and pay. this then allows you to confirm they are in attendance.
if they pre reg and haven't turned up you just dont select the rider from rider list and then they dont appear on the racers list
what we do is have pre printed slips with there info on. when they come to pay we give them the slip to check and sign. when registration closes any that haven't been checked are binned so there info isn't selcted on the computer.
seem to work ok for us but we are continuing to refine the process as we go.