Pressure plates.
Reset too quickly, if used in the simplest most straightforward construction.
You CAN build them non-resettable, but then you have to open and/or drain the whole chamber from the outside, again, or you combine it with the resetter idea from the wiki, which will AGAIN lead to the need of a second lever...
If water-triggered pressure plates wouldnt be so unreliable I'd have a few nice designs up my sleeve.
Even one that drains the room and enables access after some time has passed. Fully automated noble disposal.
edit:
I just had an idea, and it SEEMS it's working!
Ok, here's the plan:
when we build a lever we have the problem that the dwarf will actually be standing ON the lever tile, so we cannot utilize a plate to be active while we want to drown somebody. So what can we assign rooms from that creates actually an impassable tile?
that's right! STATUES!
so, we build a 1 tile wide corridor. 3 tiles deep. Last tile gets the statue, 1 tile before that is the pressure plate, civilian activated, before that is a 1 tile raising bridge.
assign the statue to the dwarve(s) to be killed.
Dwarf will sooner or later visit the statue, and stand on the pressure plate. Bridge will raise, dwarf will be imprisoned forever. Now wait until he starves or just incorporate a drownign contraption that is also triggered by that plate. I'd go for drowning, because a starving dwarf might tantrum and destroy the bridge that is blocking his escape, no?
Now the only culprit I can see right now... will the CORPSE of the dwarf still be triggering that plate? because if not, the whole thing resets upon death
AUTOMATICALLY
now excuse me, while I climax over my creativity.
Going to test build that machine right now.