The best way to deal with most pests is to be sure you have a very diverse garden that is attractive to beneficial insects that prey on your pests, like these aphids. That will never reduce your pests to zero, but it should keep them in check, so they are not so destructive. Natural enemies of the cabbage aphids include ladybugs, syrphid fly larvae, and some tiny stingless parasitic wasps.
Plants to attract ladybugs to your garden include all the Umbelliferae such as fennel, dill, cilantro, caraway, angelica, tansy, wild carrot & yarrow. Other plants that also attract ladybugs include cosmos (especially the white ones), coreopsis, scented geraniums, dandelions, calendula, marigolds, feverfew, sweet alyssum. Most of those are also attractive to the syrphid flies (hover flies) along with buckwheat, lavender, mints, bee balm, thyme , zinnia.
Grow a good selection of these mixed in with and around your garden and you will have lots of help managing your aphids and other pests! Aphids are soft bodied and are susceptible to soapy water spray. But the leaves are sensitive. It has to be soap (like dr Brunner's pure soap) not detergent (dish "soap" is usually detergent). The trouble with the aphids is they get down inside the head, so you need to pull the wrapper leaves back. Best is to treat frequently when the plant is younger, before it heads up. After the soap spray, be sure to rinse, and don't do this in direct sun.