Attempt #1: Pop off with the phoneme loss! Drop that phone[me] wherever and whenever it occurs. Memory modification, censorship, etc. Functional? NO. What changed once can change again—what was lost can be regained. Language changes, sometimes with surprising rapidity. Most likely, you've bought yourself centuries, tops.
Attempt #2: Change the biology! Surgically remove a critical sound (or 3) from the entire species and its genome. Functional? NO. When we were monkeys, we didn't have the vocal range we do now. 350
What's to say that one day, some strange mutation will give the species that sound back? A stronger solution—you've bought yourself time on an evolutionary scale, but it's hard to say exactly how long. Longer than the lifetime of your species? It's a loaded question, and one you can't say yes to.
Attempt #3: Make the entire species mute! Surely that would do the trick—homo sapiens now photosynthesizes and absorbs nutrients through its skin—no need for a mouth any more! Sign language is now the name of the game... although there are still a couple problems:
1. Still a possibility of genetic regression—or direct modification—sound, with its properties as a medium that doesn't require line-of-sight, is very handy for communication and it's understandable that future peoples might walk down the auditory road once again, whether through genetic modification to reintroduce speech or through cybernetics or magi-tech.
2. Not entirely clear about whether this fits the question or not, but I'd worry that someone working on their 'Illusions (Auditory)' or sound-box-module for purposes of communication (non-biological but still sapient communication) could accidentally 'speak' the Word.
In any case, at this point, you've bought yourself a long time. But it's not permanent, not yet.
Attempt #3, revision #2: If modification of the human doesn't suffice, what about modification of the medium? Drain out the atmosphere! Spacesuits are the new black! Better yet—genetically modify folk to survive in vacuum, or in the seas, places where uttering words with normal, atmospheric phonetics would be rather considerably difficult with human physiology and physics. However, we still are not yet free: if humanity leaves for other stars, what's to say there won't be some other world with an Earth-like atmosphere out there? (Not sure if the Word counts if spoken in climes so distant, but if it doesn't, space migration is totally the easiest solution to the issue, just saying.) And what's to say, since the atmosphere was once changed, that it can't be changed again? Drained atmosphere regained? Terraforming undone?
No, the ultimate solution, then, seems to be simply blowing up the planet and creating a society in the vacuum, living in zero-atmosphere environs without even the gravity to bang two molecules together! Prevent voyages to potentially atmosphere-having worlds with strict pseudo-religious conditioning (possession of pressurized gases is VERBOTEN AND HERETICAL), genetic modification, robot failsafes, and using the splintered bits of the solar system to craft an impenetrable shell of rubble around your nascent civilization.
And even then, it'll still fail, in the end. Without the supervision of an insurmountable, unchanging, and impartial power, the annihilation of the Enemy, or the extinction of the species, the carefully designed controls will meet their end, entropically see-sawing along the gradient of error tolerance with which they were constructed until finally the threshold is crossed and prevention is no longer possible. 350