Identifying the insect doing the damage is important. I am giving you a link to the possibilities. If you are unable to find the insect, you can try using insecticidal soap or some of the products meant for edibles. There are sections in garden centers that have products that can be used on edibles. Read the label for restrictions as to how long in between the time the plant is treated to use on your table. You may wish to submerge your picked cauliflower in a salt water or vinegar and water solution prior to using it. Any insects in the head should fall out.  https://www.gardeningknowhow.com/edible/vegetables/cauliflower/controlling-cauliflower-insects.htm