Last minute customers are THE WORST!
One of our old managers used to let people stay as long as they wanted, but luckily none of the managers put up with that now.
I keep having nightmares though that customers are in like an hour after closing and will not leave no matter what... not cool.
Honestly though there is NEVER an excuse to stay past close, especially half an hour after! There are seven days in a week and most stores are open twelve hours a day! If you can't shop with those hours there's a problem.
I wouldn't have put up with that family. I would have stood over them the whole time, maybe made up a good excuse like that I had another job I had to work at five in the morning or something. However, the fact that you're not even being paid anymore should have been good enough!