The original bolts that hold the disc on are too long, they poke about 10 mm or so into the centre of the alloy hub, there the buggers sit gathering rust, shit and and all sorts or other electrolysis type corrosion (Steel Bolts in cast alloy not good) the ends of these bolts become engorged with this corrosion and as you force them out of the alloy hub they will bring a lot of the threads with the or even worse shear off.
I used a Dremel with a flexi drive about a foot long and with a small wire brush I managed to clean a lot of the crap off the threads, you can not reach it all but if you get 70% off it will stand you in good stead. lots and lots of Plus Gas release agent for a couple of day before you start (WD40 will not do it, it lubes a thread once its turning but will not help release it) give each of the dome headed bolts a good sharp crack with a hammer and drift (Remember the hub is Alloy) and if necessary a blow lamp on each bolt before you try undoing it.
As someone said earlier try rocking the bolt back and forward once its moving and gradually ease them out.
The stupid bolts that are holding the disc are made of cheddar cheese so make sure your allen key is in good nick or even a new one. cos once it slips your down to a hammer a small chisel to drift the bolt round.
I replace all my disc bolts with shallow headed stainless steel hex head bolts that were about 10mm shorter, I copper slipped the new bolts before they went in, don't use loctite no one has ever had one of those bolts come undone, in fact one one can usually undo the foccing things when they really try.
The bolts need to be shallow headed to allow them to pass through the outer edge of the caliper.