Two options I can think of:
It will have to be drilled out, ...a PITA tbh and if you're not careful doing it then you will damage your alloy wheel as well as your hub.
Other option I can think of is to drill a small hole into the snapped bolt, ...then you will need to either tap the hole to fit a smaller size bolt or use an epoxy to fix a smaller size bolt into the hole which once set you use to unscrew the sheered locking nut.