This will only hide the 441 reply which is ERR_USERNOTINCHANNEL. You can download Jeepster's Numeric Reference to learn more about raw numerics. That link is listed on http://www.mirc.com/links.html

~ Edit ~

If I misunderstood and you want to hide other replies, you could do this...

Code:
raw *:*: {
  ; Add any numerics to the next line that you want to hide, separated by spaces.
  var %numerics = 441
  if ($istok(%numerics,$numeric,32)) haltdef
}