[Alastor Mobius Toth]

The problem with the BETA is that they don't rely on conventional logistics in same manner as humans do. Their Hives are the only command and control units, as well as their only points of resupply. You'd need to destroy a hive, and nothing can easily do that except for G-Bomb bombardment or continuous Ion Cannon barrage.

Also, BETAs can replace their numbers much, much faster than humans do, in addition to having an even greater numerical superiority. The whole point of Alternative Plans is that the conventional human strategies (including the concept of Total War) have largely failed.

Spoiler for For nature of BETA:

And in weird way, attack BETA themselves is the only way to destroy their "logistics", since BETAs are all organic mining machines that have the capacity to self-adapt (think Terraformers from X-Universe, only organic). They can also be made from any resource that contains carbon, including human corpses.

They can't be defeated in any other way than either reprogramming them en masse, or by killing every single BETA in the universe. Manually.