Window based blocking system:

To use: /block

on *:START:if (%block:pass) block $?="Please enter the password:"
on *:INPUT:*:if (%block:pass) block $?="Please enter the password:"
on *:ACTIVE:*:{
  if ($active != @Block && %block:pass) {
    block $?="Please enter the password:"
  }
}

alias Block {
  if (%block:pass && $md5($encode($1-,mu)) == %block:pass)  {
    .treebar   %block:treebar
    .switchbar %block:switchbar
    .menubar   %block:menubar
    .toolbar   %block:toolbar
    unset %block:*
    close -@ @Block
    return
  }
  if (!%block:Pass) { 
    set %block:pass      $md5($encode($$iif($1-,$1-,$$?="Enter a password:"),mu))
    set %block:treebar   $iif($treebar,on,off)
    set %block:switchbar $iif($switchbar,on,off)
    set %block:menubar   $iif($menubar,on,off)
    set %block:toolbar   $iif($toolbar,on,off)
  }
  if (%block:Pass) {
    .treebar off
    .switchbar off
    .menubar off
    .toolbar off
    window -ax @Block  
    halt
  }
}
menu @Block {
  Unblock: block $?="Please enter the password:"
}

Edited: I fixed the code