I think sock puppets and voting rings are already a thing that humans do - and will inevitably become a thing the agents do, if they find it helps their goals.
I think it’s an arms race, and therefore not winnable, and it’s a matter of economics as to which countermeasures are worthwhile.
I might previously have mentioned Slashdot’s metamoderation scheme - I was always pretty impressed by that. It’s probably not too unlike a voting plugin, but possibly with extra mechanisms. Slashdot was an early popular site which needed to tackle these issues - it’s a bit of history we can learn from, if we so choose.