dnd 5th – Can a prepared burst interrupt an attack?

Gust will repel the melee striker, but do not waste his action

You're probably talking about this aspect of the gust Cantrip:

A creature of medium or lower size that you choose must either make a save force save or be pushed up to a distance of 5 feet from you.

If you have prepared this cantrip with a trigger of the type "if a melee fighter is within range of my allies", you push it back when that happens (assuming that they failed when saving the force ).

They still have their action since they have not used it, so they can always try to attack; In fact, if they have not exhausted all their movements and are within melee range of your ally, they can simply go back and attack anyway, as if you had never done anything.

If, instead, you call the trigger the word "when they attack," the prepared actions do not interrupt their triggers, your homerun will only happen when they have already started their attack.