We don't recommend an insecticide in this situation; not only might it not work well given where the ants are nesting, but if the active ingredient happens to be systemic (something roots can absorb), it would enter the tree and might contaminate the fruit, as you noted. Ants aren't harming the plant, just using the easy-to-excavate soil to live in, though we acknowledge this is still a problem indoors. You can evict them by either repotting the plant with fresh soil or trying to flood them out. (Best to wait until the plant needs water first, if possible, to avoid over-watering the roots.) How long to flood the pot isn't something we have data for, but enough to cause the ants to abandon the nest and carry their juveniles with them should disrupt things enough they will leave on their own. An upcoming mild-weather day would be a good time to do this outside (easier than a bathtub or bucket full of scurrying ants to deal with).

If all else fails, you might be able to kill the colony by placing a bait station atop the pot, and letting the foragers collect the poisoned bait to feed with their nestmates. Follow product label directions in that case and keep any pets or children away from it. We do not know if any pesticide residues from dead ants in the nest among the roots will be absorbed by the plant, but would doubt it would happen at all or be enough to contaminate fruit, given how small the exposure would be.

Miri