Code:
on $*:action:$(/\b(slaps?) \Q $+ $me $+ \E\b/iS):*:{
  if (!%x) { inc -u5 %x
    var %slapnick = nick1 nick2 nick3 nick4 nick5 nick6
    describe $iif(#,#,$nick) $regml(1) $nick back $iif($istok(%slapnick,$nick,32),and eats the trout.,and throws the trout back into the river.)
  }
}
Replace nick1, nick2, nick3, etc..with the nicknames you want them to be slapped. (There's a space between each added as shown) Those who are not added in the local variable %slapnick will get the message as, "Casmo slaps nick back and throws the trout back into the river. Conversely, those who are added in the %slapnick var will get the message as, "Casmo slaps nick back and eats the trout."