I would try the boiling water in that area since you've not planted in it yet. I have used that in areas with no plants, and after a few full teapots of boiling water the mound was extinct.
Do-it-yourself Pest Control has all kinds of ant baits and are very informative on how to get rid of them, make sure if you have pets to get one safe for pets.
https://sfyl.ifas.ufl.edu/lawn...
