Toward the end of the war between Atlantis and Mu, such magicks were cast that their interaction rent asunder the very length and breadth and height of the world and through that tear entered The Destroyer from a place outside reality.
It was not as most thought, that the conflict of magicks shattered the realms, but The Destroyer, who stamped his hooves upon the Earth and broke it apart, that borders that were once lines upon the ground that existed only by the agreement of the people became the ends of lesser Realms, that one could no longer step across save by magic or accident. The realms of Atlantis and Mu, seperated from one-another at last, were lost, crushed out of existence by The Destroyer, who stepped from each as it fell beneath his weight into the void beneath reality.
So it was that The Destroyer stepped upon the bright green realm of Hell, and its stone cracked and bled lava, its greenery blackened, withered and died and became as ash, its bright skies became dark with everlasting clouds, its once sweet air was choked with soot and fumes, and its trees clawed at the skies in their agony. The Avatar of Summer who had ruled Hell fled to the other realms and began to search for a way to defeat The Destroyer, who sent minions to capture him.
In time, The Destroyer's minions returned and brought Adonim, the Summer Avatar, before The Destroyer as he surveyed the ruined land of Hell that groaned beneath his weight from his seat upon his throne at the pinnacle of the once-beautiful land of Hell.
The Destroyer gloated before Adonim, taunted him with the ruin of his beloved realm, and suggested that He might follow in Adonim's footsteps when Hell was crushed into the void, and step between the realms to the realm in which Adonim's people had taken refuge. The Destroyer then had his prisoner's gag removed so that he might better savour Adonim's dismay and pleas for mercy.
However, Adonim did not cry out in dismay, nor did he beg, but instead uttered the single word "[Redacted]", that by his labours he had identified as the word which had torn reality asunder. As he spoke the Word, The Destroyer, who could not exist entirely within this world's reality, was forced back whence he came as the tear closed and was held by the power of the Word... so long as it was never spoken again by any living thing.
Adonim returned to the other realms to spread the news of The Destroyer's banishment. However, Adonim warned that "should any living thing utter the syllable '[Re]' followed immediately by the syllable '[dac]' and then ending immediately with the syllable '[ted]' whether with knowledge of the consequences or in ignorance, the tear would open once again, and The Destroyer might reenter the world."
So, the peoples of the realms contemplated how the word "[Redacted]" might be prevented from ever being uttered. In the ruin of Earth, there might be various realms in which magic might fade, so no active magic could be expected to persist forever. This meant that the measure taken to prevent the Word from being spoken may rely upon magic, but its changes must persist without magic for the foreseeable future.
How can a single word of three syllables be prevented from being spoken again in its entirety? Magic may be used to effect some sort of permanent change to either the world and/or its peoples, but the change must be capable of persisting without maintenance by active magic for hundreds or thousands of years at a minimum. Answers which minimise the scope and degree of the changes necessary are preferable to those where the changes must be more widespread or drastic.
The changes must be effective regardless of technology as it is gained or lost in the realms of the world.
Technology may be considered to vary between the realms from stone-age to modern Earth, though mostly exists as some intermediate level.
The Word will have its effect if uttered on Earth or its shattered realms by any being with the potential to understand the consequences of its actions. I.e. if an infant says the word, it will have its effect, as long as it has the potential to grow up and understand that the word it said had the consequences that ensued.
A computer or a recording or a parrot uttering the Word are safe, since none of them will be able to comprehend the future consequences.
For purposes of this question, the syllables may be whatever the answerer needs them to be.
Ongoing active magic is defined as an effect that must persist in order to prevent the Word from being spoken, or enforce another change.
The laws of the universe can't be changed, though magic may make the universe act as if its laws had changed while the magic remains active.