Code:
on @*:text:*:#: {
  if ($nick isop #) { return }
  set -u3 %fld. [ $+ [ $chan ] $+ . $+ [ $nick ] ] $calc( %fld. [ $+ [ $chan ] $+ . $+ [ $nick ] ] + 1)
  if (%fld. [ $+ [ $chan ] $+ . $+ [ $nick ] ] >= [color:red]5[/color]) {
    ban -ku60 # $nick 11 Please don't flood
  }
}


And change 5 to the number of messages allowed to send before flood.