ummmm, had a case of that once :( (Not me mind!)
when I was at college we went on a trip to Chessington, and one of my mates, he wasn't huge, but a fair size and very tall as well. He had been desperate to go on Ramesis Revenge for ages, we all got on and it went round once ok. Then they stopped it and ove the microphone asked for the large boy in the front row to get off as he was stopping the safety bars closing properly! He was so embarassed, and I felt so sorry for him, it was horrible :-(
I mean ok, fair enough if he is too big, but dont let the ride go once with the bars not closed properly, and then announce it to everyone on the ride and watching to hear that he needs to get off!