WorldGuard's blocked-cmds and aliases

Hi,
I made custom command with BetterAlias plugin, but in a region I want to block /portails !
Region block the /spawn and /portails but player CAN DO /portails ...

I want to block it in the region !

My code doesn't work !

Code:

    @EventHandler(priority = EventPriority.MONITOR)
    public void Blockage(PlayerCommandPreprocessEvent event){
        String[] array2 = event.getMessage().split(" ");//Single whitespace
if(array2[0].equalsIgnoreCase("/portails")&&event.getPlayer().getWorld().getName().equals("Mecraft")){
                event.setMessage("/spawn");
                event.getPlayer().sendMessage(prefixMecraft + ChatColor.RED + "Maintenant fais /portails !");
                return;
        }
    }

It logs that :
[13:15:48] [Server thread/INFO]: fantakill issued server command: /portails
[13:15:48] [Server thread/INFO]: [BetterAlias] [BetterAlias] §bRunning console command for fantakill: /warp portails fantakill
[13:15:48] [Server thread/INFO]: CONSOLE issued server command: /warp portails fantakill

I want player do /portails in the region be changed into /spawn and as they can't do that, it is not allowed ! Else it tp the player to spawn.
If this is not the solution, how to do please ?
/!\ I don't want to block /portails in the whole world ! Just in the region where /spawn is not allowed too /!\
+ someone sais "dispatchCommand" is not correct... I don't know why but he told me to use "PlayerCommandPreprocessEvent"

Help please