Derived from b&, derived from band, derived from banned.
The & sign is known as an "ampersand."
Used because it sounds better than "bampersand" and when someone decides to be cute and correct mods/ops on which word to use.
Regular: Where is RegularX?
Op: I used /banpersand on him.
Regular: @Op */bampersand
(Regular left chat "Banned: No, it's /banpersand")