Is there a way of getting rid of ants that's safe for my cats?
We had ants last summer. They came in through the kitchen and a wall near the kitchen. We caulked it up so they couldn't get in, and the problem was solved.
This summer, we've had ants again. They primarily come in between the areas we caulked last summer (can't get to it with the caulk gun because of the radiator), and go after the cats' food. We've taken to only putting their bowls on the floor while we feed them, and then taking them away. This generally reduces the ants, but if we go away for a weekend and have to leave some food out, we get a lot of ants.
We've read/been told that poison (especially poison the ants will take back to their nest) is really effective, but we're concerned about using such a solution in our apartment because our cats like to cause trouble and get into things, and we don't want them playing with any poison.
What is an effective, cat-safe way to get rid of ants?
When you need to leave food out for longer periods of time, you can put the food bowl in a pan of water so that the ants can't reach it. Of course the pan should be small enough that the cats can eat without getting their paws wet.
The easiest way to get rid of the ants is to remove their food source, which you have done. In addition, you could inject some sort of ant killer behind the baseboards and in other crevices. Borax is probably a good choice; it is not safe for cats in quantity (e.g. to use in a litter box), but minute amounts should not cause problems. Some people report success using soapy water, vinegar, or even catnip!
There are also products that contain ant bait/poison inside a plastic case, with openings big enough for an ant to get through but not a cat's paw. As I recall, these look rather like those round stick-on air freshener thingies. Assuming your cats are not in the habit of eating ants, these should work well and be safe.