Rejecting to get credit back

I have set up a study, but participants get an error message on the first screen and only spend a few seconds on there. Still they seem to be registered as participants and uses up my money. I think I have found the error (it is about the participant ID). I don’t seem to be able to reject these participants unless I activate my study, which I don’t want to do since anyone signing up will keep getting this error message. How can I fix this and “clear up credits on my account”?