Bukkit

locale:
  deny: You are not allowed to go back from the nether!

onewaynether.bypass

import org.bukkit.World.Environment;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.HandlerList;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerPortalEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class OneWayNether extends JavaPlugin implements Listener {
	public static final String LOCALE_DENY = "locale.deny";

	public void onEnable() {
		this.getServer().getPluginManager().registerEvents(this, this);

		this.getConfig().addDefault(LOCALE_DENY,
				"You are not allowed to go back from the nether!");
		this.getConfig().options().copyDefaults(true);
		this.saveConfig();
	}

	public void onDisable() {
		HandlerList.unregisterAll((Listener) this);
	}

	@EventHandler
	public void onPlayerPortal(PlayerPortalEvent event) {
		Player p = event.getPlayer();
		if (p.hasPermission("onewaynether.bypass") || p.isOp())
			return;
		if (!event.getFrom().getWorld().getEnvironment()
				.equals(Environment.NETHER))
			return;
		if (!event.getTo().getWorld().getEnvironment()
				.equals(Environment.NORMAL))
			return;
		p.sendMessage(this.getConfig().getString(LOCALE_DENY));
		this.getLogger()
				.info(p.getName()
						+ " tried to get from the nether to the normal world but was denied.");
		event.setCancelled(true);
	}

}