When something is extremly WRONG and unnacceptabl, illegal, bad, perverse, etc.
For example if someone likes the idea of massaging their cats udders, that would be BANNED!
Person 1: "Hmmm I like Rupert Grint, his ginger hair is so sexy!"
Person 2: "Arg, thats wrong, ginger hair is banned!"
Person 1: "Hmmm doesn't my cat look sexy licking her buttons"
Person 2: "That's BANNED!"