I'm surprised that watering didn't drive them out (usually does the trick), and even more surprised that the ant powder hasn't worked, so I've no idea what to suggest, I'm afraid.

Seems to be a plague of them this year.....

Can you find the nest without damsging the roots too much, maybe you're not getting to the eggs

At this time of year they're difficult to shift because they're almost at the point where the exodus will occur, when those eggs they've nurtured for so long turn into winged ants and leave.

If the pot isn't enormous, and you can find a container big enough for the rootball to sit inside and be covered in water, take the plant out of its pot and stand it in the larger container full of water, weight the rootball down so its completely immersed, and leave it for 4-6 hours. After that time, wiggle it around in the water a bit, gently, then extract it, inspect the root ball, and repot using fresh compost as necessary. And this time, stand the pot on something to keep it off the ground - pot feet or bits of roof slate, anything to give a bit of clearance beneath. Dust beneath with ant powder periodically.

You could wait to do this till after the flying ants have gone - it'll be a lot easier then, and that should take place within the next four weeks, on a warm and sunny day.