We have ants on the balcony. We also have ants on the allotment (farming the aphids, mostly, which is both impressive and really, really annoying, leading as it does to the death of the broad beans). I have, therefore, been seeking ways to get rid of ants.
The executive summary seems to be: you can't; learn to live with them. I have been trying this for some time, but the depredations are getting to be just a bit much. (Especially as they seem to have killed off the worms in the wormery as well.) So I've tried a few things.
I'm not prepared to do boiling water; plus it would take ages to boil enough with the storm kettle on the allotment, and on the balcony, it would kill whatever plant was in the relevant pot as well.
On the allotment, the best solution without a doubt has been ant nematodes. The compost heap was absolutely swarming with the damn things before I applied these, as was the paving by the pear tree; both are now clear. I also tried it on the balcony, but with less conclusive effect; the satsuma tree (which seemed to be the worst affected pot) looks to be mostly clear now, but they've just moved to the potato box.
Flooding out is one option (if they've built their nest in a pot where the plant won't mind that). After emptying most of a watering-can into the potato box, I very soon saw lots of frantic ants carrying away eggs. But where to? I fear I may need to excavate the Area Under The Herb Table. I'll repeat the treatment on the potatoes again shortly (and the potatoes should do well for it, as well).
Another suggestion I've seen a lot is cinnamon. So last night I went out and sprinkled cinnamon in copious quantities all over the balcony. Curiously, I couldn't actually see as many ants anyway as I had before, so maybe the drenching has sent them off to find a nest somewhere that isn't my balcony. I'll report back on the cinnamon in a couple of weeks.