So, I hate hate hate useless migrants. At first I was tempted to use the magma approach, but I realized something. That would kill most if not all of the migrants, and I kinda want to keep that extra miner, mason, ranger, or etc. So, I devised a plan...
A room over a chasm (Not preferred), bottomless pit (Not preferred), magma pipe/pool (Okayish), or Underground lake (Preferred). The room's floors would be made of hatches, and there would be a tiny alcove with a lever in it. The lever would lock the door. There would be two other levers outside the room somewhere, one to unlock the door, and one to make the floorhatches disappear...
The plan is to set up everything, and then with the lever that locks the door, set its profile to only accept the unwanted migrant, and then order to pull the lever. The jolly old dwarf would go and pull the lever, locking itself inside. Then, the lever to open the hatches would be pulled. The dwarf would either: Fall to its death (Bottomless pit or Chasm), which would be boring, burn to death (Magma pool/pipe), which would be okay, but kinda fast and not very amusing, or fall into the water of the underground lake and start drowning while all the snake/olm/etc. men would rip the dwarf to pieces, which would be very amusing indeed...
Then you would order the hatch lever to be pulled to close the hatches, pull the lever to unlock the door, and send in another migrant, if needed.
Variations could include multiple levers, each for a certain dwarf, which would make multi-deaths plausible...
So people, tell me of your ways to get rid of migrants, which can be like this, which only gets rid of one, or possibly more at a time, or gets rid of them all at once.