Removing spinning 1/2" 5 piece in Sandstone?

So I encountered a bolt today that I was hoping to change the hanger on or replace. When attempting to unscrew the hex head bolt, the cone seemed to be spinning in the back of the hole with the center bolt itself. I'm assuming that the bolt is rusted to the cone very tightly and the rock that it is placed in was soft enough to simply groove out deep in the hole so that they just spin together rather than actually unscrewing. Has anyone encountered this before? It seems like it could be really difficult to get this thing out. Would some kind of tuning fork between the rock and the hanger for added pressure while unscrewing work? Some kind of lubricant? Rock is sandstone. 

Try clipping a draw or sling to your harness and to the cold shut and leaning out on the bolt as hard as you can and unscrewing at the same time. Sometimes you can lean out on it hard enough to get the cone to engage at a different spot in the hole and you can then unscrew. Better would be some gentle funking to get the cone to engage but might be sketchy funking a worn out cold shut.

The rock being sandstone, placing anything between the rock and the cold shut might scar the rock, but if you can't get it out with Josh's suggestions here are a couple of suggestions which work, but may scar the rock.  If you are going to re-use the bolt hole and put a Real(TM) hanger on it, the scaring might not matter if kept to a minimum.

Get a cold chisel and drive it between the cold shut and the rock.  Now try to turn the bolt.  If the bolt does start to loosen you might need to keep driving the cold chisel deeper to maintain force.  Alternatively, you can use a crow bar(2' works well) and drive it between the cold shut and the bolt.  You can just lean on the crowbar as the bolt unscrews to maintain the force.

To minimize scarring you might try to get the chisel or crowbar between the outer edge of the cold shut and the washer on the bolt, but it is easier to damage/bend the bolt. Hey, but it's a 3/8" bolt in a 1/2" 5-piece!

Per Josh's suggestion try using a Funkness device to reset the cone in the hole. Careful that you don't funk your elbow and/or shoulder. It's going to be difficult to avoid creating a nasty rock scar if you resort to applying a chisel or crowbar, but if you're serious about removing the bolt and the funkness fails you'll need to apply some outward pressure while unscrewing the bolt. If you can get some space between the rock and the cold shut an angle grinder will safe you time and aggravation. Good luck.

A "tuning fork" with a 3/8 slot made from an sds chisel bit is a very effective tool for this kind of problem. Of course also great for Rawl drive compression bolts from 1/4" to 3/8".