This might not be the best forum. There are sprays to kills or repel the little buggers-commercial and homeopathic. I don't know one ant from another so I'm not much help. I feel for you, though.

Spraying the plants with insecticides will kill pretty much everything on them--including caterpillars, and the ants will come back anyway. I recommend Amdro or another similar ant bait. They pretty much only kill ants and they get to the queens which shuts down the entire colony. There are many types of ants, however, so I don't know if this will work on, say, carpenter ants or some other types. We use it very effectively for fire ants here in Florida.

You can spray the ground with horticultural oil, which they sell in most large stores, and that will kill most of the insects that crawl around. I use that on gardenias, and camellias to kill white flies and other insects. I don't worry about caterpillars on these plants, since I don't know any butterflies that use them.

I've used Amdro around my house to try to keep carpenter ants out. Never seemed to affect my caterpillar activities.

Fourteen years ago, I used Dursban to solve a problem like yours. Fumes did not kill insects so I could apply it around the plants/insects I was trying to protect. Dursban is long gone. Don't know if something similar exists these days.

Trying to get rid of one insect while keeping another is definitely a rock/hard place situation.