Nothing because the effect of each blast is instantaneous. The effect of the spell is dealing damage (and pushing), once a blast hits, it resolves and disapears. In my opinion what you're proposing is casting dispell between one blast hist and the other is cast, so no, you cant dispell the casting of the second blast. Is that what you ment?
That is the question, but the instantaneousness of the effects is irrelevant.
If you ready an attack for "if the warlock hits me with a spell", and the warlock casts eldritch blast, you get hit by beam #1, then the trigger ("hit with a spell") has resolved so the readied action goes. If you kill the warlock, the rest of the spell never happens.
The spell itself is in some way present during that interval. If instead of attacking, your readied action was a dispel targeted at The Spell, I would probably let the third level spell cancel the cantrip. It wouldn't
undo the previous blast, because that's an instantaneous effect and has already taken effect, so there's no magic to dispel, but I wouldn't let you continue throwing beams.
Okay, better example, since the dispel wording is weird (and strictly speaking, as written, the spell has no effect at all on any magical effect that is not "a spell" on a thing, so there's no reason for the "magical effect" to be listed as a targetable thing). Let's look at antimagic field. "I ready an action, to cast antimagic field if the warlock hits anyone with a spell." I'm standing next to the warlock. He hits someone with eldritch blast. I now cast my antimagic field. What happens? It seems to me that the other blasts can't happen; magic is suppressed inside the field.
Basically, I think you're reading too much into "cannot be dispelled" here; I think the point of that is just to articulate that you can't use dispel magic to remove the fire created by a fireball. It's not to handle the exceedingly weird special case of getting to have an action between parts of the resolution of an "instantaneous" spell.
