The Investigators can block the Gate that allows Abhoth into our world with an Elder Sign.
Abhoth was brought into the world. From the book Dunwich: Return to the Forgotten Village:
Long before the advent of mankind, beings came to this place to make use of the natural magical energies found in the valley. The Mi-Go used these natural rifts between worlds to open a vast gate far below the surface, allowing a great, alien being to partially enter this world.
This being was later known as Abhoth, Source of Uncleanness...
Abhoth can be driven back by dealing enough damage to it to drop it to zero hit points.
Abhoth (from stat block):
No weapon using kinetic force can harm Abhoth permanently. Abhoth regenerates from all kinetic damage at the rate of 20 points per melee round. Fire or magic will cause normal damage. If Abhoth is reduced to zero hit points, it withdraws and sinks far below the earth where it is inaccessible to further damage. It oozes again toward the surface after healing from its injuries.
Assuming that Abhoth can be driven back through the Gate, if the Investigators can cast the Elder Sign, the Gate would be sealed.
Elder Sign (from the core rulebook 7th edition):
An Elder Sign may be formed into a leaden seal, carved in rock, forged in steel, etc. When made active beside an opening or Gate, it makes that path unusable to minions of the Great Old Ones and Mythos Gods, as well as to those greater beings themselves.
Terrors of the Secret War, a source book for the game Achtung! Cthulhu, backs up the techniques outlined above.
Because it is native to our dimension and dwells on our planet, Abhoth cannot be banished or truly killed; it was found or created here by the elder things, and used to fashion the first life on Earth. As a result, I fear there are only two ways to “defeat” Abhoth. The first is to strike some sort of a deal with it, as already mentioned. The second is to block its subterranean gates, sealing them with Elder Signs to prevent any spawn from coming through.