Sure you can. How big is it? My 1-1/8" Crosby shackle cost $70. I wouldn't cut that if I can avoid it. We'll see this week. *grin*
Have you tried spraying it with something like PB Blaster and wrapping it in newspaper and plastic wrap for a couple of days? Hit it with a hammer and heat up the bow of the shackle. A crowbar and a big pipe wrench for leverage and you should be able to break it free.
Holding the shackle steady to cut through it will be just as big a pain.
Had an anchor shackle that would not budge even with PB Blaster and leverage. Found a Dremel with a cutting blade made it a quick painless five minute job. THE BIANKA LOG BLOG: REMOVING A FROZEN ANCHOR SHACKLE AND SWIVEL
I would recommend using a reinforced cutting disk. But, I used a regular cutting disk and it still worked fine. Just bring extra disks in case you break one or two.
I have used a hack saw to cut and remove many rusted and of no value anchor shackles. I used locking vice grips to hold the shackle in one place while I made the cut. With sharp blade the soft galvanized shackle cuts like butter.
Two huge pipe wrenches. The trick is to somehow imobilize the shackle. If I can get the whole mess into the dinghy, I jam one wrench under my leg. If not, then I jam the shackle wrench on the mooring ball, so I won't turn. Then crank on the wrench on the pin.... Might be a good Idea to put lanyards on the two wrenches!
A long object like a crow bar goes through the shackle the pipe wrench then spins the shackle against this breaker bar.. If you can bring a 3' pipe that will also fit over the handle of the pipe wrench now you've got enough leverage to break most anything free... Never not been able to get one apart with this method and I help to decommission about 15 moorings per year..