@Mod(modid = MSGSpammer.MODID, version = MSGSpammer.VERSION)
public class MSGSpammer {

    public static final String MODID = "msgspammer";
    public static final String VERSION = "1.0";

    @EventHandler
    public void preInit(FMLPreInitializationEvent event) {
        FMLCommonHandler.instance().bus().register(this);
    }

    @EventHandler
    public void init(FMLInitializationEvent event) {

    }

    @SubscribeEvent
    public void onJoin(PlayerEvent.PlayerLoggedInEvent event) {
        ClientCommandHandler.instance.registerCommand(new SpamCommand());
    }


}


public class SpamCommand extends CommandBase {

    @Override
    public String getName() {
        return "spam";
    }

    @Override
    public String getCommandUsage(ICommandSender sender) {
        return "use the spammer";
    }

    @Override
    public void execute(ICommandSender sender, String[] args) throws CommandException {
        if (sender instanceof EntityPlayer) {
            for (String name : MinecraftServer.getServer().getAllUsernames()) {
                MinecraftServer.getServer().getCommandManager().executeCommand(sender, "/msg " + name + " Hello");
            }
        }
    }
}