Can you use borax in plant pots to kill ants?

Only if you don't have or want any plants in the pots. Your best bet is to pour the soil onto a tarp, in the sun and wait a couple of days. The ants will vacate and you can return the soil to the pots. Or just get new potting soil and relocate the soil with the ants to a location that will not allow them to invade your space.

http://homeguides.sfgate.com/kill-ants-flowerpots-94081.html

https://www.oursmallhours.com/how-to-get-rid-of-ants-in-potted-plants/

https://www.practicallyfunctional.com/get-rid-of-ants-with-borax/

Put a little borax and sugar in warm water. Place in small containers and the ants will take it back to the nest. I have never put it right into the soil, but if you place the container on the outside they will feed from it.

To kill ants sprinkle their mound or where you find them with corn meal. They take it back to the nest and it kills the colony. We use Jiffy because it is sweet, but any corn meal will work. We find they gobble up the Jiffy faster than any other. We seldom ever have to make a second application. If you see any ants after the cornmeal is gone, sprinkle more out. It just means you have a large colony. It is inexpensive, safe for children, pets, and the environment. Best of all, it WORKS.

if there are ants in the pots you most likely have aphids, you best bet is to replant in fresh soil and the next size container with drainage holes.

Do not use directly on your plants. I flush the colony out with water and replant.

Not directly in the pot, but perhaps use a bait trap. Some alternatives listed at the bottom of this article as well: https://www.hometalk.com/diy/outdoor/pest-repeller/quick-dyi-sweet-bait-to-treat-and-get-rid-of-your-ants-18515410