According to the help file, using /background with the -x switch, should remove the background from the specified window.

Code:
on *:JOIN:#:{
  if ($nick == $me) {
    background -x $chan
    var %win = $+(@,$chan)
    window -hp %win
    drawtext -o %win 11 Tahoma 30 3 5 $chan
    drawsave %win $+($chan,.bmp)
    background $chan $+($chan,.bmp)
    .remove $+($chan,.bmp)
  }
}


In this case, it doesn't, why?