Just had a post like this on another forum and the options where
1. heta it up and use mole grips.
2. hammer and chisel to force it out.
3. Drill a whole in it and use an easy out
4. Force a bigger bit into it.
5. Cut a section through it and use a big screw driver/impact driver.
6. Drill the head off and once the disc is off use mole grips on the shank thats left.
Spike has covered most of the options
One other option that i do before any drilling/cutting is to use a centre punch and a hammer make sure it is sharp to dig into the outer face of the cap screw then hit it on an angle towards the left to undo it easier than using a chisel and should dig in better so it does not keep slipping bit of heat will help to crack it loose used this method lots on machines at work & cars/bikes etc
If that does not work i would recommend drilling the head off to get the disc off then you can get the rest out after as no 6 above or file two flats on the stud so you can put a spanner on it.
Removing Broken disc bolt.
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