You can buy a clutch cage tool to make the clutch static while you turn the bolt. Of course, if the engine is still on the bike engage a high gear and you are good to go. IF the cylinders are out, put some pieces of wood over the crankshaft and put a thick piece of metal on the connecting rod, thus avoiding it to "sink" into the crankcase.
I was going to replace the camchain, so I just let the crankshaft swalow it until it got jammed and then I was able to remove the bolt.
Raul