When the ants make their way up onto your insect-infested lemon tree, it's rather like a herdsman attending his cattle on a mountain pasture. That's to say, the ants "milk" the insects and care for and defend them by drinking and collecting the sugar-rich excretions of the insects. The insects, in turn, suck the plant cells dry, after which they set about sorting their way through the "plant soup" that they have sucked in, and because they have a high reproduction rate, their primary area of interest is the proteins that it contains. They then simply excrete the surplus sugars. What the ants are really interested in, however, is the energy-rich waste. So what can you do now?
Well, what you need to do is stop the ants from making their way into the pot from the outside. For example, you can place the citrus pot on a tray that you should keep full of water, and then place this on a wooden block to prevent the water from saturating the citrus soil.
If the ants have nested in the pot, I would advise repotting the plant. If you do so, get rid of about a third of the soil from the plant ball and, as far as possible, any ants' nests that you can see. The rest of the plant ball should then be submerged completely in water at ambient temperature for about half an hour to drown the ants.
As a "first-aid" measure you can apply grease bands to the base of the citrus plant to prevent the articulated insects from scrambling their way higher up.
To be absolutely correct, it should be pointed out, finally, that you are only allowed to tackle species of ant that don't come under a nature conservation order.