When the diamond ore is removed, the gravel falls down and interrupts the redstone signal.

When a player walks down the hallway and removes the diamond ore, the gravel placed on it falls down. This destroys the redstone on it and the piston retracts.
Now lava placed on it can flow down and traps the player.

The front side of the tunnel has to be covered, of course.
Also, the lava should be further away from the diamond ore, because the lava flows very slowly.

Instructions:

First, place a block of diamond ore (or other natural block that is safely mined) at the end of the tunnel, and a block of gravel (or other natural block that falls down) above it.

Now build a (activated) Redstone track over the gravel block.
The redstone track must lead to a piston that pushes a stone under a lava spring.