Protecting cabbage from pests
To discourage soil borne pests from the cole family (cabbage, broccoli, etc), crush egg shells into the soil around the plants.
Air-borne insects, including the cabbage butterfly are kept away by bitter, strongly aromatic herbs close to the plants. The most potent is wormwood. Tansy is possible, but it will take over your garden. It is also forbidden in several US states because of its toxicity to cattle. Chamomile is milder, but also helps. If you neglected to plant these herbs in time for this year, you can still lay them on top of the plants during the growing season.
A solution for people who like to fuss
Strained, ground-up bugs will make a spray to repell more of the same, but no one seems to have tested the method. Spraying has to be repeated after every rain. The herbs only need to be planted once.