“Stop, I’m serious” should work the first time. If your friends ignore it, that tells you something.

A version of stop exists that everyone understands, and it does not need to be repeated or justified. When a group keeps going past it, the issue is not that they missed the tone. It is that your no is being treated as part of the game. How people respond the first time tells you where you stand with them.

Say it once, plainly, and then go quiet or leave rather than escalating. If the same people need telling every time, it is worth thinking about who you spend your lunch with.