You could have had block all, block selected or block none for your dialog. In the menu you can shorten the on and off to one line with a $iif statement like:

menu nicklist,channel,query {
  iCub's PM Blocker
  .$iif((%pmblock == on),Off,On): $iif((%pmblock == on),unset %pmblock,set %pmblock)
}

Also not really a big difference but you can use only one on dialog like:

on *:dialog:af:sclick:*:{
  if ($did == 1) {
    /window -w %qnick
    .msg %qnick I Suppose Someone Has To Be You're Friend
    /dialog -x af
    /window -a %qnick
    /unset %qnick
  }
  elseif ($did == 2) {
    close -m %qnick
    .msg %qnick Fuck Off I Dont Want To Talk To You
    /dialog -x af
    unset %qnick
  }
  elseif ($did == 3) {
    close -m %qnick
    .msg %qnick Sorry.. I Don't Like You
    /dialog -x af
    unset %qnick
  }
}

Overall it's a good dialog, good job.