Place a block such as Wool or Diorite somewhere.
Place a Board that says "Free Diamonds below!!!" on the block.
Dig a hole in front of the block with a pickaxe and spade until you reach the bedrock.
Then, either fill up the very bottom of the hole with spikes or wait in the hole (you need will really strong equipment such as Diamond gear and food) for other players to enter, then kill them.