Start in forge mode with at least 2 players.

Have one player get in a vehicle (note: it's much more fun if the other player doesn't know what you're going to do).

Then after he gets in, go into edit mode/ball mode and get close enough to the vehicle so that the green hand pops up on your reticle.

Then press Y to make the vehicle disappear.

When it does, the person driving it will be forced to respawn.