If you don't want to go organic... spray pesticide on the stems.
If you want to go organic... sprinkle diatomaceous earth on the dirt around the plant. A good, thick ring will work well, and won't harm your plant at all. But if you water, it will wash away, and you'll have to reapply. Or you could use the old aphid trick... not sure how well it works on ants, but will probably at least ruin the flavor for them. Mix 1 part food-grade liquid soap (dishwashing liquid) with 10 parts water and stir gently to mix. Put in a spray bottle and spray liberally over the plant.