Ok so i decided to make a plugin that changes the unknown command message and allows you to show an actionbar message. With this plugin you can also block commands via the config.yml file, we are going to implement a reload command so you can reload your config.yml file but currently if you have plugmanx you can reload the plugin with that or you can just restart your server. /reload could work but it might break the plugin. You can change the messages in the config.yml
To get this to sync with the vanilla commands aswell you need to go to your spigot.yml file and change your unknown command message to this "unknown-command: '&cThis command does not exist.'"