Original: http://www.hawkee.com/snippet/5733/ || i edited the parts for message received and if wasnt sent that got blocked.

Commands:

/addnick

/delnick

/shownicks

Alias AddNick {
  if ($1 isin $read(acceptednicks.txt, s, $1)) { 
  echo -a 7 $+ $1 is already on Accepted Nicks | halt }
  else { write AcceptedNicks.txt $1 $1
    echo -a 7 $+ $1 added to Accepted Nicks
  }
}
Alias DelNick {
  if ($1 isin $read(acceptednicks.txt, s, $1)) {
    write -ds $1 AcceptedNicks.txt 
  echo -a 7 $+ $1 deleted from Accepted Nicks | halt }
  else { echo -a 7 $+ $1 is not in Accepted Nicks
  }
}
Alias ShowNicks {
  if (!$window(@AcceptedNicks)) { window -znkg1 @AcceptedNicks }
  var %x $Lines(acceptednicks.txt)
  while (%x) { //echo @AcceptedNicks $gettok($read(acceptednicks.txt,%x),1,32) | dec %x }
}
on ^1:OPEN:?:*: {
  if ($read(AcceptedNicks.txt, s, $nick)) { msg $nick 7*The Message has been sent 4 to the user please wait for a reply. 7(3This could take a 5while12) }
  else { msg $nick 7This is a PM Blocker.9Please Contact me in a channel, or tell me to add you to my list, Thank you. | /halt
  }
}