This should work. At least from my testing it did work.
I'll also mention, this will still keep them locked from the command to every 30 seconds to be safe from them alone spamming. (Even with the 10s cooldown on the command itself.)

Code:
 on *:TEXT:!highcard*:#: {
  if ((%floodhighcard) || ($($+(%,floodhighcard.,$nick),2))) { return }
  set -u10 %floodhighcard On
  set -u30 %floodhighcard. $+ $nick On
  var %randdeal = $rand(1,13)
  var %randplayer = $rand(1,13)

  if (%randdeal == 1) { set %dealer 2 }
  if (%randdeal == 2) { set %dealer 3 }
  if (%randdeal == 3) { set %dealer 4 }
  if (%randdeal == 4) { set %dealer 5 }
  if (%randdeal == 5) { set %dealer 6 }
  if (%randdeal == 6) { set %dealer 7 }
  if (%randdeal == 7) { set %dealer 8 }
  if (%randdeal == 8) { set %dealer 9 }
  if (%randdeal == 9) { set %dealer 10 }
  if (%randdeal == 10) { set %dealer Jack }
  if (%randdeal == 11) { set %dealer Queen }
  if (%randdeal == 12) { set %dealer King }
  if (%randdeal == 13) { set %dealer Ace }

  if (%randplayer == 1) { set %player 2 }
  if (%randplayer == 2) { set %player 3 }
  if (%randplayer == 3) { set %player 4 }
  if (%randplayer == 4) { set %player 5 }
  if (%randplayer == 5) { set %player 6 }
  if (%randplayer == 6) { set %player 7 }
  if (%randplayer == 7) { set %player 8 }
  if (%randplayer == 8) { set %player 9 }
  if (%randplayer == 9) { set %player 10 }
  if (%randplayer == 10) { set %player Jack }
  if (%randplayer == 11) { set %player Queen }
  if (%randplayer == 12) { set %player King }
  if (%randplayer == 13) { set %player Ace }

  if (%randdeal > %randplayer) {
    msg # The Dealer wins flipping %dealer over $nick $+ 's %player $+ . Too bad.
    if ($nick !isop #) {
      msg # .timeout $nick 30
    }
  }
  if (%randplayer > %randdeal) {
    msg # $nick wins flipping %player over the Dealer's %dealer $+ . Hooray!
  }
  if (%randplayer == %randdeal) {
    msg # $nick and the Dealer tied, it's an amazing feat with all them cards in the mix. PogChamp
  }
  unset %dealer
  unset %player
}


Reason for unsetting at the end. For some reason, I rolled 3-4 ties when I had it do var %dealer and var %player.