Finally i got this working.

No more falling through chairs. And even if you fall no Nope kick after
dismount.

src/com/cnaude/chairs/Chairs.java

@@ -116,11 +116,18 @@ public class Chairs extends JavaPlugin {
     protected void reSitPlayer(final Player player)
     {
     	player.eject();
-		sit.get(player.getName()).remove();
+    	final Entity prevarrow = sit.get(player.getName());
 		Block block = sitblockbr.get(player.getName());
-        Entity arrow = block.getWorld().spawnArrow(block.getLocation().add(0.5, 0, 0.5), new Vector(0, 0, 0), 0, 0);
+		final Entity arrow = block.getWorld().spawnArrow(block.getLocation().add(0.5, 0, 0.5), new Vector(0, 0, 0), 0, 0);
 		arrow.setPassenger(player);
 		sit.put(player.getName(), arrow);
+		Bukkit.getScheduler().scheduleSyncDelayedTask(this, new Runnable()
+		{
+			public void run()
+			{
+				prevarrow.remove();
+			}
+		},40);
     }
     protected void ejectPlayer(final Player player)
     {

src/com/cnaude/chairs/PacketListener.java

@@ -17,6 +17,7 @@ public class PacketListener {
 		this.pm = pm;
 		this.pluginInstance = plugin;
 		playerDismountListener();
+		falledPlayerDismountListener();
 	}
 	
 	
@@ -50,6 +51,31 @@ public class PacketListener {
 	}
 	
 	
+	private void falledPlayerDismountListener()
+	{
+		pm.getAsynchronousManager().registerAsyncHandler(
+				new PacketAdapter(PacketAdapter
+						.params(pluginInstance, Packets.Client.ENTITY_ACTION)
+						.clientSide()
+						.listenerPriority(ListenerPriority.HIGHEST)
+						.optionIntercept()
+		)
+		{
+				@Override
+				public void onPacketReceiving(final PacketEvent e) 
+				{
+					if (!e.isCancelled())
+					{
+						Player player = e.getPlayer();
+						if (pluginInstance.sit.containsKey(player.getName()))
+						{
+							pluginInstance.ejectPlayer(player);
+						}
+					}
+				}
+		}).syncStart();
+	}
+