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|This page in a nutshell: If you tell people not to do something, the people will do the opposite of your advice.
Don't stuff beans up your nose is a metaphor for not telling people to not do something. This is because telling people not to do something makes them want to do it more. Don't give them ideas.
It comes from a story about a mother who was leaving her son at home for the afternoon. He was a troublemaker, so she gave him a long list of things not to do. "Don't play with the oven. Don't break anything. Don't let any strangers in the house. Don't try to start your father's car. And don't stuff beans up your nose!" This gave him the idea to stuff beans up his nose, and that is exactly what he did.