Assuming that the Quori Monolith that someone linked to in the comments is accurate, critically that
These towers ... stand anywhere between 250 to 500 feet tall with a diameter between 100 to 120 feet wide. While the walls of a Quori Monolith are 10 feet thick and the material is psionically reinforced (hardness 30, 1,440 hp per section), ...
... the base is always ringed with eight, equally spaced open archways ...
In order access any of the Quori Monolith's abilities, someone must be access the Quori Monolith Control System at the top of the tower and make a Use Magic Device check. ... Any result of a natural 1 on the Use Magic Device check, except to activate it, results the immediate some of all effects and starts a self-destruct sequence.
Hit it with a stick
There are rules in place for damaging objects; if you hit a monument with a big enough stick (to overcome its impressive hardness) for long enough (to remove each section's remarkable number of HP), it'll come tumbling down. If there are 8 archways, there are 8 solid bits between the arches; determine how many can be removed before the tower simply falls over (I'd suggest 3).
Hit it with (a) magic (stick)
Disintegrate or Psionic Disintegrate can both remove significant portions of a building in short order; a couple of castings should be enough to remove enough of the base to cause the tower to tumble.
Shatter is not, strictly, a viable option (the tower is far too heavy), but you may rule that individual portions of the tower can be targeted individually.
... or sunder a fully-charged Staff of the Magi, but that's even more expensive ...
Press the Self-Destruct Button
The tower has a built-in self-destruct function: roll a nat-1 on a "do something" attempt. Have someone keep trying to "do stuff" until they roll a nat-1.
Intentionally triggering the self-destruct appears to have a UMD DC of 35.
Of course, either of these options "automatically alerts any quori within 10 miles", so there might be a fight to prevent the quori from saving their tower.
Put a Portable Hole into a Bag of Holding (or vice-versa) and rule that the result destroys the tower or sucks it into the Astral Plane.
All of that is irrelevant
You're the GM; the towers are exactly as easy to destroy as you want them to be.
Is there a McGuffin of Monolith Destruction (or re-programming) that the players need to find? Build a quest to acquire it.
Are the towers well-guarded? Build a giant melee for the PCs to overcome as they chip away at the tower.
Does the self-destruct Summon defenders? Build a "hold this position for X time" encounter, possibly with an element of chance in when, exactly, the tower destroys itself (be very careful with this; the PCs are likely to be very weak when the tower goes pop).
If this is something that the PCs want to do, you're in a great position: they've already given you a hook for the next leg of their adventure. With just a little bit of prodding, they can even be conned into telling you what kind of leg that should be.