You'll have to get cunning, and wait till you can set up a more substantial fortress. You can circumvent their immunity to cage traps with a cave-in, or retracting bridge, but I worry they'd be too big for it to retract with them on top.
Set up a little tunnel on the other side of the walls, and carve fortifications into them so they can see through. You'll lose the stoneworker(s) that carves them, but keep a pet dog or something chained up back there behind some green glass windows and make sure they have LOS at all times with more fortifications to reinforce their goal of reaching this dog behind glass windows.
Now, they will attempt to path all the way around, hopefully, and in the process you can drop a long section of floor out from under them with a support linked to a lever, knocking them into cage traps, and hopefully capturing them.