From The Martial Arts Encyclopedia

BBJ is a common typographic error (typo) seen on many Martial Arts forums. In most cases it seems to be a simple "fat-fingered" error when the user is intending to type "BJJ", which is short for Brazilian Jiujitsu.

However, it's possible that it's meant in a derogatory sense in some cases, putting the emphasis on "BJ" which is slang for oral sex and referencing what some traditional martial artists profess is a homosexual subtext in grappling styles.