Three green, yellow and red stuffed bell peppers in a row like a traffic light.
Green is mild (0 stars), yellow is medium (2 stars), and red is so hot that most people can't finish it.
The plate is on the house if the guest can finish the red one by themselves.
Capsaicin, while able to cause an unbearable spiciness when eaten, takes time to have an affect on your body. Someone could very well just chug the red pepper and suffer the consequences later. You would need some kind of gimmick, like after someone has finished the red pepper, place an ice cold glass of milk in front of them and start a timer. If they can last X amount of minutes without needing the milk THEN we can say they 'won'.
What if someone found they were able to tolerate (even enjoy) the red pepper and came to the restaurant every day for a free dinner? It could start costing you money.
It would have to be one per person. I guess you could enforce that by having a hall of fame with photos of the winners.
Don't give that shit for free.
Make them pay...always