As I explained in the DBAG HoF two weeks ago, this is a mischaracterization perpetuated by someone on the board who thought I 'yelled' at them for saying someone's old user name. The quote at the link I provided and knapplc's response underneath is a pretty succinct explanation.
tl;dr: Don't use someone's old user name (or even their current one, for that matter) as a punch line for a joke or a means of insulting/bullying them in the main part of the forum. It could be construed as a personal attack/trolling/flaming depending on the context.
As far as I can tell, doing so in The Woodshed is fair game.