The solution is to make your players afraid of a well-practiced DM that knows how to fuck with powerhungry wishes.

"I wish for an invincible unassailable fortress"

it appears... and then falls directly on top of the player

"I wish for true immortality where nothing can kill me ever ever ever"

granted... in a pocket plane with no way in and no way out

"I wish the final boss was already dead""

player is time-skipped to a far future where the boss is dead and so is everything in the campaign they know or care about

To ensure the factor and keep the wishes reasonable, be sure to insist that the wish magic must be spoken out loud, without pause, and more or less be a single sentence like in almost every example given of wishes in antiquity fiction.