If she is too young to understand why some rules exist, she will be impervious to either of your trying to talk it through, to physically prevent it, or even to physically punish her. She will not understand either. What she will get out of it, though, is that she doesn't own herself in one or another way. Even not involving sexuality, this is a dangerous feeling to have.
Unless she is already ill, a dirty nose is just unaesthetic, but not actually dangerous.