We've just come up against this problem too. Someone rezzed an item that they had been offered by an unknown avatar, declined (twice), and yet it was still in their inventory. When rezzed, it could not be taken back or deleted, and even as landowner, i couldn't return it.
They tried my advice of deleting all scripts inside the linkset and creating a new script that had an llDie() in the touch event, and couldn't even get that script to run.
Finally, they got the advice here about wearing it to work. It went back into their inventory, but the 18 prims are still used up from our allocation, and an object area search can't find it in-world.
Is this the old Ghost-prim issue with a new twist ? Should somebody raise a fault on this?