I'm sorry, but an All you can eat buffet is exactly that unless stated otherwise.
If you don't want me to sit there and eat as much food as I possibly can, then don't say it's an all you can eat buffet.
Charge people by the hour at that point.
If it's $10 a person, charge $8 an hour. People think they're getting a deal, until they stick around for a while and get their $100 bill for 3 people for 4 hours.