Is there a euphemistic word to tell guests that they cannot take outside food after dining in the buffet restaurant, and that they feel embarrassed to take outside food?

The word "forbidden" is a bit like the army and does not conform to the sense of service.

First of all, you must be polite and have the word "please" in your words. In this way, customers will not be emotionally rebellious and lose customers. If the word "you" is used, it should be euphemistic.

For example: This restaurant welcomes you to enjoy the buffet, but does not support takeout. Please support us. Honest and trustworthy people are welcome to continue to enjoy delicious food next time.

This information must be conveyed to every customer without making those who have not committed the crime feel suspicious.

Mark this dining agreement, similar to a contract, in a conspicuous place, or send it to every customer before dining to ask for their signature. There are also hardware, monitoring, some people are good at stealing and packing them in plastic bags after eating, and even professional thieves, you still won’t lose money.

These are pre-event control, then in-event control, supervision and guidance at any time, and then ex-post control, which summarizes experience on problems that arise and improves management level.