Why didn't they do something about Ahkmou?

[Ghid]

Welcome to the Boards.

Per BioSector01:

Before the Matoran dumped all of the infected koli balls, Ahkmou fled, leaving only a sign at Ahkmou’s stand saying “Gone Fishing.” After some time, Ahkmou later returned to Po-Koro. At this point, Turaga Onewa knew that Ahkmou was allied with Teridax.

After the Bohrok Kal were dealt with, it is implied that Ahkmou was kept under close watch:

Ahkmou continued to work as a shopkeeper in Po-Koro, and did not misbehave again.

As for why… Only Greg can ultimately answer that, but a possible answer might be the fact that the Matoran were dying left and right to everything for a thousand years, and anything harsh enough to put him out of the way for good (including the death penalty) would lose them a valuable pair of hands, and Ahkmou was an exceptionally skilled carver.