If you have good signage that tells them they must push the button to stop charges then I would not provide a refund more than once per customer. When I get things like this I take the information and send them a
token or
tokens and keep the information on file under their name. So in a case like this the question is how much do you refund them? If they did not push the button you really have no way of knowing how much time they really used. I would probably send them a short note with a couple of
tokens and let them know you are sorry they had the issue and you are happy to give them the
tokens while they learned how the system worked but there won't be any refunds after this time.
Just my two cents worth....next time they will either push the button or use cash.