Was it irremediable? I believe that it was said they'd need multiple reincarnations until the corruption could be 'filtered' out of the soul by the Pattern.
Or, now that you mention it, perhaps this was the case for Trollocs, but Myrddraal were irredeemably corrupted?
RJ said they couldn't be reborn as anything but Shadowspawn because they're tainted. It's thus irremediable as every incarnation they'll ever have again will be as a tainted being. He didn't distinguish between type of Shadowspawn IRRC, or he spoke of Trollocs in his example but didn't exclude the others.
He compared that to Nym. An existence as a Nym doesn't taint a soul, simply "borrows" it for an incarnation, after which it goes on being reborn as a human.