If I have an Ezreal, Prodigy in play and I rebuttal (Mel's signature) something, can I use Ezreal to pay the additional cost on rebuttal? So it would be 1 and a power for stealing a spell?

No, Ezreal, Prodigy does not reduce the [A] cost on Rebuttal.

Ezreal specifically reduces optional additional costs — costs that use both the phrase "as an additional cost" and the word "may," like Repeat (Rule 356.2.b.1, FAQ #8415). Rebuttal's "You may pay [A]" does not say "as an additional cost." Instead, it's a cost within an instruction — it's paid on resolution of the spell, not during the process of playing it (Rule 742.1). Ezreal can't touch that.

So stealing a spell with Rebuttal will still cost you the full [A] even with Ezreal on board.