If you want to allow a one time only thing to happen for a given nickname, you can use a variable:

Code
on *:text:!GKill:#Kill-A-Gnome:{
  if ($($+(%,gkill,$nick),2) == 1) { msg $chan you already used !gkill }
  else {
    msg $chan Gkill used! boom
    ...
  }
}

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