Cut a slit in it and use a heavy duty screwdriver. I'm guessing the bold has probably siezed on anyway, have you used any penetrating oil spray to loosen it up?
You might just have to drill out the bold head and use pliers to pull out the thread. Pliers might work on the bolt itself if you can get a good purchase.
Other options include welding on a new nut to the bolt and using this to gain leverage.
I usually find drilling it out and retapping it the best course of action. If it has sheared then there is little use in trying to remove it as it has rusted in to start with. I have tried other methods even welding a stud etc and that has sheared too. No substitute for drilling and tapping in my opinion then using coppaslip when re-assembling.