Every few summers we have problems with ants in our kitchen (also northern California). This year we got 'em too.
I've tried lots of things. This is what works for me.
1. Ants come after food, so don't invite them to the feast. Clean counters, don't leave dirty dishes out, keep food in sealed containers, etc.
2. Buy a small spray bottle of some kind of poison. I use Ortho Home Defense because that's what they had at the store, but anything will probably do. NOT aerosol but a bottle with a pump sprayer, like a Windex bottle or Fantastik. Use it very sparingly on walls, baseboards, outside of cabinets (i.e. AWAY FROM FOOD OR COUNTER SURFACES), where you see a line of ants. The bottle says '12 Month Protection' but I find it's more like 12 hours. 8^< Then again, I only use a tiny bit, just enough to dampen the surface.
3. For food and counter surfaces, use peppermint oil. Get a little bottle of peppermint oil (Whole Foods has all kinds of 'essential oils'). Take the cap off and the neck of the little bottle has a plastic plug with a tiny hole. Turn it upside down and hit it on the counter and it leaves a little drop. Do these in a line, then use a finger to smear them together into a solid line (I use a spiral motion to widen the line. It doesn't have to be very thick, just a smear.) Do this around the edges of your counters and sink and ants won't cross that line for several days. You need to re-apply when you begin seeing ants again.
Or put 2 or 3 drops in a cup of water in a spray bottle and just lightly mist the counters, cupboards, etc. (You have to shake it up vigorously before you spray and during spraying because the oil doesn't mix with water very well.) You also have to do this every couple of days. It doesn't get sticky or anything and it makes the kitchen smell nice!
3. Diatomaceous earth. I haven't tried this but I have friends who swear by it. It's a white powder, very fine like flour. You sprinkle it around the edges of your counters. I haven't tried it because you can't use it on vertical surfaces, and that's where I see the most ants.
Of course wherever you see ants you can spray them with water (or your peppermint oil spray) and then wipe them up with a paper towel.
Ants are not 'dirty'. They don't carry disease like rats or mosquitoes. Ants live underground where there are all kinds of nasty little fungi and bacteria and parasites, so their little paws exude an antimicrobial agent. If an ant walks across a piece of food, it's actually cleaner than it was before! So you're not worried about germs or whatever, just the nuisance of the ants themselves.