Disallow chunk creation.

[Taugeshtu]

how do i overwrite chunks

Well, that's where things are complicated
We can't access chunks directly in lua (hmmm... Can't remember exactly why not, but it was some kind of multithreading-related stuff)
But we can use cBlockArea or whatever it's called exactly! There's where math kicks in: you got to calculate your cBlockArea cuboid for given chunkX, chunkY coordinates.
And I don't have any example yet, because I wasn't touching it still. Though I will be soon(-ish), but that's not guaranteed.

But you don't need to overwrite whole chunk just to cancel its generation! Just return true (?) with OnChinkGenerating hook if you need in to be done with current chunk.