If the stud has broken off at the clamp, there will be a bit left protruding from the head. Use lots of "penetrating oil" a good 24hrs before attempting next nuts !
You could try mole grips or a stud extractor, but these methods are only 50:50 sucessful usually  but worth a try.
Best bet is to weld a nut onto the broken stud & try and remove. The welding super heats the stud and can help to release it.
If they shear flush, the head MUST come off and a pillar drill used to get the buggers out. If they are 6mm studs, first drill a 3mm then up to 4mm or 4.5mm and you should be able to clear out the stud bits remaining.
Last resort (best method) is "spark errosion" but it ain't cheap
Good luck
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