Pathfinder allows players to "ready" their action so that they act "just before" the action that triggered it (more info here). For example, a spellcaster can wait to cast a counterspell when another caster tries to cast something (source).
A spellcaster can even use Dispel Magic as a counterspell. It would seem that a spellcaster, then, would be able to ready their action to dispel an instantaneous spell, such as Fireball. Unfortunately, Dispel Magic adds this in its description text:
The effect of a spell with an instantaneous duration can't be dispelled, because the magical effect is already over before the dispel magic can take effect.
Presumably, Dispel doesn't prevent spells from being cast, and Dispel only works against spells that have already been cast. By the time Dispel could be used as a counterspell, the instantaneous spell has completed.
Other abilities (e.g. Spell Disruption) seem to have similar wording.
It is unclear (to me) whether an instantaneous spell could be disrupted in any other way.
Are there any mechanisms to interrupt instantaneous spells?