Ugh. I feel your pain. I've built an underground corridor that leads from my stockpiles to my newly built outpost/bunker system/fort in the SE corner. This corridor meets a channel that I can fill and drain with water at the pull of levers, and with another level I can open the corridor to the channel, flooding it pretty quickly. Excellent goblin-be-gone ploy when all else seems lost, and way more cool than just raising a bridge.
Some time ago, my fortress was besieged by goblins, and the SE fort fell to the enemy. With all my military manpower already committed, I decided to employ the "Flood the hallway" tactic to keep the goblins from flooding into my fortress. Only problem was that one of the doors wouldn't close, what with the limestone boulder jamming it open. No prob, I think, I'll designate it for dumping. Everything outside's been forbidden, should be easy enough for them to do.
Nope. No one was interested. No matter what I did, they simply refused to move the rock. One of the haulers actually ended up standing dead still, two tiles away from the thing, with "No Job" listed as his assignment. I was fuming. In the end, no goblins came down the corridor after all, and I moved the boulder by building a wall out of it, but it still annoyed me big time.
I think, in hindsight, that the problem was that the dump zone was outside, and that they were not allowed out there, but I'm just not sure. Either way, never have I wanted a simple RTS-style "click a unit, then its target" system than when this happened.
"Hey, you! Urist McDumb***!"
"Yes?"
"Pick up that rock!"
"OK, I picked up the rock."
"Wonderful, you'll get far. Now move over there, without putting down the rock."
"OK. And now?"
"Now put the rock down, without injuring your feet. There. Good boy. Now you can stand there and look dumb again."
But no, that would've been too easy, wouldn't it?
(Yes, I know, it's still in alpha)