There is no "proof" when it comes to history - all we have is the balance of evidence. Every decent piece of evidence we have available points to Atlantis (at least the version that became widely known and survived into the modern day) being a piece of fiction created by Plato as a rhetorical advice.

Evidence:

Atlantis appears in Timaeus and Critias, works by Plato, as an allegory and express fiction.

There is no earlier reference to Atlantis we have ever discovered.

All references to Atlantis we are familiar with from the time period after Plato seem to be referring directly back to his work.

Plato making Atlantis up is about as close to a fact as you can get, historically speaking. It is the most likely situation based on what we know, and there's no evidence to the contrary.