Two things, the ^ and the haltdef to prevent the default display.
Code:
on !*[color:blue]^[/color]:join:#:{
  echo # 9 $timestamp * $nick Is4 # 9binnen gekomen.
  [color:blue]haltdef[/color]
}
on !*[color:blue]^[/color]:part:#:{
  echo # 8 $timestamp * $nick Heeft4 # 8verlaten.
  [color:blue]haltdef[/color]
}
on !*[color:blue]^[/color]:nick:{
  var %i = 1
  while $comchan($newnick,%i) {
    echo $v1 15 $timestamp $nick Heet Nu $newnick
    inc %i
  }
  [color:blue]haltdef[/color]
}
on !*[color:blue]^[/color]:quit:{
  var %i = 1
  while $comchan($nick,%i) {
    echo $v1 15 $timestamp $nick <Message for quitting>
    inc %i
  }
  [color:blue]haltdef[/color]
}