From previous posts on here over the years you need to really heat up the bolts to stand any chance of removing them, but obviously not everyone has the right tools, know how or patience to do this so what should really be a simple job can turn into a complete nightmare.
I've been through it myself, when only one of the six bolts came out about half way then just snapped level with the hub, and due to circumstances at the time where i just needed the bike back on the road it was quicker to buy another wheel and get the tyre changed than to give the wheel to my mechanic to try and sort out but then maybe get told a couple of days later that it was either not happening or the threads had been damaged trying to remove the snapped bolt.
It's due to heat transfer when using the rear brake, as the disc is bolted directly to the wheel, rather than having a floating outer section like the front discs, which prevents this from happening. The two different metals of the bolts and the wheel get fused together making them a proper pain to undo, which is even worse the first time as i think whatever thread lock they originally used in the factory was some super duper strength stuff.
And it certainly doesn't help matters with the high chance of the allen key bolt heads rounding off easily as well.
If you do get the bolts out and they have even the slightest bit of damage or rounding off then i would really suggest replacing them so that you have half a chance of removing them next time. I bought myself some spare bolts after last time so i've got them to hand if needed, and there's no need to go all gung ho with the thread lock glue either, just a tiny bit will do and avoid using the extra strong stuff, but make sure they are all torqued up correctly, as over tightening will just further the pain in the future.
I noticed i'm gonna be due a new disc in the not too distant future so as i'm reading your woes it's making me wince thinking i've got it all to come again soon.
My MOT is in early September though so i might even just buy a new disc and get it done properly at the same time, 'cos at least they'll have everything needed to hand if it becomes another stubborn bolt scenario.