Ultrasounds won't work, I don't even know if they can hear them. I would recommend a simple recipe: neem oil, black soap like Marseille soap. If you're in the second week, it shouldn't be a problem. A treatment once a week until the 4th week would help eliminate them. If you see any at the end, you can always wash your plants at harvest time. It seems to me, but I might be wrong, that spiders appear when the air is too dry.