Clearly the elves need to be punished and shown humility. For this, we need a trap that incorporates all things dwarven: Magma, grievous injury, more magma, plenty of irony, sadism and magma.
Construct 4 tiles wide hallway. Make it very long. One end of the hallway has 4 floodgates (controlled with lever, magma-proof), holding down a LOT of unpressurised magma (or slightly pressurised, just in case the magma is too slow for your liking). The other end leads outside. Construct weapons traps, equipped with blunt weaponry that won't instantly kill the elves, but *will* grievously harm them or break bones (magma-proof weapons will make reseting the trap less tedious, but might make the traps too powerful). Place them randomly (or if you don't want to put things to chance, make about 2-4 unavoidable lines of em) inside the hallway. On the other end of the hallway is a 1 tile wide tunnel, leading to the surface outside. Put a door/bridge/other locking & unlocking mechanism on that part of the tunnel, again magmaproof. Dig a hole 2-3 stories above the hallway, next to the floodgates. This is where you will drop the elves from into the hallway.
Once the hallway is built, the fun can begin. For best effect, try to capture the elves as they are retreating from a siege (so they try exit the map rather than attack), although regular prisoner elves will do too.
1) Drop your prisoners down the pit into the hallway (make sure the hallway is locked at this point). This will land them near the magma (still held by floodgates) and probably break several of their bones due to the fall damage.
2) Open both the floodgates and the exit at once. The magma will be released, and the elves will run for their lives down the exit. Those who are too slow due to broken bones/injuries/unconciousness (caused by the fall) will be baked.
3) While running, they will probably get hit by the hammer traps, breaking additional bones (thus slowing them enough for the magma to catch up).
4) If an elf managed to get past through the hallway, quickly lock the door behind him (or use pressure plates do to it for you).
5) All the slow elves will be burnt to a crisp, while the survivor will flee for his life and live to tell the horrors you inflicted upon them to his foul kin.
6) Cut down a tree for every elf killed this way. Decorate exit of hallway with remains.
For extra laughs, put plenty of wooden weapon traps in the hall (you'll have to replace them though).