Would something like Tanglefoot around the base of the pot work? It's a physical gooey barrier that will just keep them out of the plants but not really affect the ants otherwise. I don't know if your species of leafcutters are beefy enough to get through it.

I would definitely look into exclusion instead of extermination, though. Easier with non-flying critters.

They're pretty big here, but that’s a good suggestion. I suspect if I make it difficult, they’ll just find an easier way to other plants. I’m more than OK with that.