Go to options and change the "drop" to The left control button.

Now go to a vanilla server (not sure if this happens with bukkit) and in a filled spot in your hotbar, press Left control, you should drop a full stack of items.

Now go to an Empty slot, and press Left control, you will be disconnected from the server. If you keep joining and doing this over and over, you won't be able to connect to the server for a while.