Water boils at 100 degrees and becomes steam, so unless you have a pressure cooker (which is really unlikely in the case of the groups we're talking about) you aren't going to hit that.
Even if you could, prions aren't something you can sterilize effectively short of incineration (at which point you're eating ashes rather than meat) - it's the same reason why meat from cows that have had mad cow disease (variant CJD) can't simply be marketed as "cook thoroughly before eating".

