It might have been changed in Dig Deeper, but this guy definitely can destroy constructed walls. I didn't see him destroy any constructed floors, but that might've been luck.
I got him. FINALLY, I got him.
I was really ticked off, too. He just wandered around randomly destroying everything he could touch — workshops, farm plots, furniture, doors, floor grates. Anything constructed or built, he knocked down.
But oh, how satisfying it was to finally get him.
Okay, here's what I did.
After he ate his way through my dining hall one table and throne at a time — 44 of them in all — I decided to do basically what Silhouette suggested. First I waited for him to destroy everything he felt like destroying. When he decided to hang out in my archery range for a while, I laid down a field of cage traps in my once-and-future dining room. (Fortunately I had a ton of mechanisms and cages lying around.)
Then I built a cave-in trap. I was very lucky that my dining room just happened to be over unmined ground. If it'd been over a mined-out area below, I'd have had to shore up the lower floor with supports. And now that I think of it, I'm not sure that would've worked.
Anyway, the dining room was over unmined rock, and beneath my meeting hall. I picked a good spot in the dining room and built a support. Then I tore up the constructed floor of the meeting hall (it was constructed metal floor over rock) and channeled out a square. So I had a 3x3 area that was supported only by the support on the level below.
I rigged up a lever to the support. (Oh, and I had to put a grate down so an idiot noble whom I couldn't recruit and manually move could get off the square area. Stupid noble.)
Then I baited the trap. I built two marble tables and two marble chairs and set them up in the dining room. The support was by the door, then there were cage traps, then the furniture.
Once I saw him head for the tasty snack I'd set out for him, I paused and used the step-ahead-one-frame feature to keep from missing anything important. When he stepped onto the cage traps, I had a dorf pull the lever. The support deconstructed, the 3x3 section of floor collapsed into the dining room, a huge cloud of dust erupted (in retrospect maybe 3x3 was overkill), and when it cleared, Omo Onoos the black dragon, Omo Onoos the uninvited houseguest, was snoring peacefully inside one of my cages.
Yes!
Of course, about a dozen dorfs are also now snoring peacefully inside cages. And there's a gaping hole in the meeting hall floor. And the dining room was basically destroyed and wants rebuilding. And I'm not sure if my poorer dwarves will survive the season without starving to death, since I was counting on those now-destroyed farm plots to feed them. And I've got about a dozen workshops to rebuild, and and and.
But that son of a bitch is behind bars where he belongs.
Man, it feels good to win.
(On preview: Grendus, there's no Dwarf Companion for the Mac, unfortunately.)