When you say "reinfested it", does that mean they're making a nest in the compost? Or that they're running all over the leaves and stems? If the former, you will have to turn the pot out, get rid of any ants in there, repot in fresh compost in a cleaned pot, and this time, stand the pot off the ground on pot feet or bits of broken roof slate or whatever to give clearance between it and the ground. If the ants are running over the foliage and stems, spray for aphid infestation.

.....or Scale insect:-)