If you don't want there to be a way back to Athas, then there is simply no way for the characters to get back the Athas.
In previous editions, Spelljammer and Planescape went into detail on how travel was possible between worlds. It was almost trivial to travel between Faerun, Krynn, or Oerth.
On the other hand, other settings were deliberately described as difficult or impossible to reach. Athas and the Ravenloft demi-planes were of this nature.
As a suggestion on how to set-up your one-time trip to Athas:
Have the characters come across an ancient portal. Perhaps the landscape surrounding it is a slowly growing area of sickly and dying plants. This area could be starting to encroach on fertile croplands, or perhaps the "worthless" land has been granted to a character and he wishes to break the land's "curse".
When the characters cross to Athas, the portal goes dead. A portion of their adventures in Athas will be a quest to re-activate the portal.
Once the Athas episode is done, the characters activate the portal and cross back to their world. Whenever one of the characters enters the portal, all the other characters are also pulled through. Once back in their world, the portal once again goes dead, this time also crumbling to dust.
With the portal gone, the land is no longer having its life drained away, and it can recover its fertility. Also, with the portal gone, there is no other way to travel to Athas.