Main class:

package me.samut8.AntiProxy;

import org.bukkit.Bukkit;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.plugin.java.JavaPlugin;

public class AntiProxy extends JavaPlugin
{
  public static AntiProxy plugin;
  @Override
  public void onEnable()
  {
	Bukkit.getPluginManager().registerEvents(new Listeners(), this);
	getLogger().info("Anti-Proxy 1.2 by samut8 enabled!");
    saveDefaultConfig();
  }
  public void onDisable()
  {
	  getLogger().info("Anti-Proxy 1.2 by samut8 disabled!");
  }
  public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
	  if(cmd.getName().equalsIgnoreCase("antiproxy")) {
			  onDisable();
		      onEnable();
		      reloadConfig();
		      sender.sendMessage("§cAnti-Proxy 1.2 config reloaded!");
	      }
	return false;
  }
}


Listeners:

package me.samut8.AntiProxy;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Scanner;

import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.player.AsyncPlayerPreLoginEvent;

public class Listeners implements Listener {

private AntiProxy plugin;

	  @EventHandler(priority = EventPriority.HIGHEST)
	  public void onAsyncPlayerLoginEvent(AsyncPlayerPreLoginEvent event)
	  {
	      try
	      {
	    	String playerip = event.getAddress().getHostAddress();
	    	String player = event.getName();
	    	String url = "http://www.stopforumspam.com/api?ip=" + playerip;
	        Scanner apiscanner = new Scanner(new URL(url).openStream());


	        Bukkit.getLogger().info("Checking if " + player + " (" + playerip + ")" + " is trying to connect with a proxy...");

	        if(apiscanner.findInLine("ip yes") != null)
	        {
	            Bukkit.getLogger().info(player + " (" + playerip + ")" +  " tried to connect using proxy!");
	        	event.disallow(AsyncPlayerPreLoginEvent.Result.KICK_OTHER, plugin.getConfig().getString("Kick_Message"));
	        	for(Player p : Bukkit.getOnlinePlayers()) {
	        		if(p.hasPermission("antiproxy.notify") && plugin.getConfig().getBoolean("Notify")) {
	        			p.sendMessage(ChatColor.GRAY + "[Anti-Proxy] " + player + " (" + playerip + ")" +  " tried to connect using proxy!");
	        		}
	        	}
	        }
	        apiscanner.close();
	      }

	      catch (MalformedURLException exception)
	      {
	        exception.printStackTrace();
	      }
	      catch (IOException exception)
	      {
	        exception.printStackTrace();
	      }
	      }
}