Any creature reduced to 0 or fewer hit points by this spell ...
The disintegrate spell isn't doing anything to the troll's hit points, it's only stacking on nonlethal damage.
This raises an interesting question, though... Does a regenerating creature automatically become stable when reduced to negative HPs?
It stands to reason that it does, but i couldn't find RAW on this...
If they only took non-lethal damage, then they aren't actually below zero. You track your subdual damage completely separately from your real damage, and when it exceeds your current hit point total, you're unconscious. So they could take 10000 non-lethal damage past their hit point total, and still be alive but unconscious, and slowly recover from it via their regeneration.
This is why in 2nd edition the Ring of Regeneration was ungodly... I remember once in a game a character literally fell off of a high cliff, took thousands of points of damage, and then woke up a week later and was fine.
If they took enough
real damage to knock them below zero, then they'd start to bleed out like anyone else.
The bigger question is what happens if a creature with
Fast Healing gets knocked below zero... as Fast Healing says it works like "normal healing", but faster. Normal healing doesn't work at all when you're below zero. That has its own special rules.