Generally guys like this are factored into the business plan, but sometimes people try to outsmart (or in this case outeat) the restaurant.

I remember reading once that crab legs were often a loss leader food. The restaurant offered crab legs to pull people in despite being too expensive for the buffet. Most people ate a few and then filled up on cheaper foods. Some people however would just eat crab legs.

I remember another example of this where a customer stayed from lunch until dinner. Not only did lunch cost less, but now that's two meals. The fix there was a three hour table limit. Basically affects no one, except the exceptions.

They have restaurants where people who eat less are charged less and those who eat more are charged more. It's called a la carte.

There are also buffets where you can be charged based on how much you eat , they're called "buffet by weight" or by the plate.

If you don't want people to eat all they can eat, don't open an all you can eat restaurant.

I would say that they should be required to let him finish his meal, but they can refuse to let him back again.