If the bolts are spinning and not coming out there's two possibilities. Either the bolts are stripped or the receptacle in the roof has broken free in the roof allowing it to spin in it's hole.
If the bolt is stripped then it will be about an even pressure to turn it the whole way around. You can try putting an upward pressure on the bolt by wedging something under the racks foot and prying it up a bit while you try to back the bolt out.
If it's the receptacle, as you spin the bolt you'll feel it catch slightly. This is the flats on the receptacle catching the flat sides of it's hole It'll snag six time in one rotation; six flats on the receptacle. How to get the bolt out: You can either put something under the rack foot and pry it up popping the receptacle out. It won't take much force. It will leave ugly hole in your roof.
Or a longer but cleaner process. Drill the head of the bolts away, leaving just the shafts in the rack feet. Lift the rack up and off your roof exposing now eight bolt shafts poking out of their receptacles. Cut the bolt shafts off and grind away the tops of the receptacles. The problem with this is the bottom portions of the receptacles are going to fall down into the roof of the truck. And they'll stay there unless you pull your headliner down to get them.
I recently(today) just repaired my roof rack after it tore off my roof.
The thread is here,
http://www.tahoeyukonforum.com/forum/showthread.php?t=26768
I'm going to update it tomorrow with some pictures as to how I fixed it.
