There is a way to use the ampersand (&) by using it with combinations of letters and shopping it on top of pictures to get words like "S&" on top of a desert or "L&" on top of a field. The most common usage for using the ampersand is to create the word banned with b&. This meme started on Something Awful.
B& or b& is the most commonly used form of this combination on 4chan is B&, in response to a post someone has made (illegal, ban-able). For illegal activities, especially posts of child porn.
An intentional typo for B& (seeing as the & and 7 characters are the same key), not in common usage.
underage b& is a common use of the b& term. It refers to an underage poster being identified as underage and therefore will be banned due to the site's minimum age limit.
V& or Party V&, a reference to the 4chan Party Van, has become common.