Command Blocker?

Hi, i been creating some custom plugins lately and i am stuck on this one no idea on how to make it like i want it to be so that if a player were to do a command for example:
/plugins it will block them saying something like "Sorry, Only Operators Can See This" or something anyway you guys can give me like a full set up on how to code it or just send me the whole coding

 

Code:java

@EventHandler
public void onCommandPreProecess(PlayerCommandPreProcessEvent e){
  Player p = e.getPlayer();
  if(!p.isOp()){
    if(e.getMessage().equalsIgnoreCase("pl") || e.getMessage().equalsIgnoreCase("plugins")){
    e.setCancelled(true);
    p.sendMessage(ChatColor.RED + "You're not allowed to see the plugins!");
    }
  } else {
    String message = "";
    int index = 0;
    for(Plugin pl : Bukkit.getPluginManager().getPlugins()){
    if(index == 0){
    message = message.concat(pl.getName());
    } else {
    message = message.concat(ChatColor.GRAY + ", " + ChatColor.AQUA + pl.getName());
    }
    index++;
    }
    p.sendMessage(ChatColor.GOLD + "Plugins("+index+"):");
    p.sendMessage(message);
}