Without digging out all my stuff one of my heroes has an ability that can deny surges. When does that surge denial allowed to be triggered? Is it when the pool is gathered or can they deny you after you decide where to spend your surges?

I think I would follow the "Golden Rules (p. 18) instruction :

" Timing conflicts may arise when two or more players wish to use an ability with the same triggering condition. In these situations, the current player (the player who is currently taking his turn) decides the order in which the abilities are resolved."

So the attacker would decide if the defender denies the surges before of after he decided their use.

It's hard to tell without the exact wording of the ability (even knowing if it was from an item, or a particular hero or class would help here). It is most likely going to be that it's when the pool is gathered. The person spending the surges spends them all at once, and I believe (but my memory isn't perfect) that those abilities say (or imply) that they cancel them on/after the die roll (not after the spending of surges, which is a later phase of the attack round).

If timing conflicts resolve in favor of the active player then I would think the OL in question here has two options. The OL can let the heroes deny him the surge before he spends them or let them take a surge after he spends them, potentially messing up all of his work (say something requites two surges to use when you could have used a one surge ability on something else).

Regardless, that skill has to be dealt with.