First, a word about pots - if the pot you have is much larger than the pot the rosemary's currently in, pot the rosemary into something a bit bigger, but not something too large. When it's grown on enough, then pot into the large container. The reason for this is that a small plant in a large pot means the compost within is largely unoccupied, which leads to souring and the proliferation of pathogens, or at least conditions which your rosemary won't like.

Otherwise, you can empty out the pot, including ants; or soak it thoroughly with water so that its saturated - this should mean the ants carry off the eggs elsewhere, but you may have to keep on soaking it. They don't like very wet conditions. I suggest also that you should use new potting compost in your pot for a permanent plant.

That's useful info Bamboo; I often wondered about the potting on regime and thought the plant just felt lost in too big a pot. Now it makes more sense.

Yea, if it looks lost in its pot, it's too small for the pot is a rough guide...

Thank you for your advice. We'll give it a try. Interesting to know why not to put a small plant in a large pot.