NNS has already got a lot of good slaps ... lol

You can even shorten the script further by placing those slap messages, including the $1 identifier line by line vertically, in a txt file called, say, 'slap.txt' and then play them to the targeted person on the channel.

alias slap { describe # $read(slap.txt) }

This way a random line from slap.txt will be played upon the trigger of slap alias. Of course, more messages can be added in the future.

Shorten the script.

Only one /describe command is needed to randomize the %randmsg

alias slap {
  var %randmsg = slaps $1 around a bit with a large trout.:slaps $1 with a fish.:slaps $1 with a pair of sunglasses.:bitch slaps $1 $+ .:slaps $1 with a birthday cake.
  describe $chan $gettok(%randmsg,$rand(1,$numtok(%randmsg,58)),58)
}

Changing

  set %randmsg $rand(1,6) ——   unset %randmsg

to

 Var %RandMSG $Rand(1,6) 

Will eliminate a line of the code that'll be un-needed :o.

No need for a global set variable here.
Use a local variable to avoid harddrive writes and reads.
ANYWHERE $chan works the short form # will work.
The main reason $chan exist is for doing more elaborate
things like $chan(0) $chan(#).topic etc etc
$r(1,6) is short for $rand(1,6)
This should be in a nicklist popup because of $1

menu nicklist {
  Random Slap $$1:slap $1
}
alias slap {
    var %r $r(1,6)
    if %r == 1 { describe # slaps $1 around a bit with a large trout. }
    if %r == 2 { describe # slaps $1 with a television. }
    if %r == 3 { describe # slaps $1 with a fish. }
    if %r == 4 { describe # slaps $1 with a pair of sunglasses. }
    if %r == 5 { describe # bitch slaps $1 $+ . }
    if %r == 6 { describe # slaps $1 with a birthday cake. }
}