Try putting Diamacious Earth in the holes when you plant next time. How old is your garden? It seems to get better as your garden gets older.
If you have wood ash , make a slurry of it and water and dip the root ball in it before planting.
Make barriers out of cards or paper for the neck of the plant.
I did the same thing last year when they were so bad here, removing big scoops of soil from my garden. I think there must have been 50-75 maggots per plant. Funny I have seen none of them here this year and my fall crop was not infected. I think it was like using the first crop as a trap crop. After that it got better.

A number of things will help keep the small, grey flies from laying the eggs that become the maggots. Start with floating row covers or tar-paper around the plants stems. Wood Ash, Diamotaceous Earth, hot pepper, or powdered ginger ,ounded around the plant stems are deterents. Parasitic nematodes can aid in controlling the maggots.
The best way is the floating row covers supported over the rows by plastic hoops with the edges well buried so the flies cannot get in.

Instead of tarpaper, I switched years ago to making squares of inner tube rubber (from bike tubes, or ask at any tire place for a blown tube). I make about 3 inch squares, and cut with a sharp scissors (like Fiskars) into the middle from one edge, then make several cuts about 1/2 inch long radiating out from the middle to allow the stem to expand. Put it on at ground level, slipping it around the plant stem so its centered, when transplanting the cabbages out to the garden. I've used this system successfully for many years--failures only when it is knocked off the plant.
Brad Sondahl

Here is a link that might be useful: my garden pages

This year I built a structure made of plastic tubing and plastic ropes on top of my cabbage, cauliflower and broccoli beds - all together 25 plants. My beds have a wooden frames. The structure looks like a box without walls connected to the frame and it is wide and long as the bed are and about 4-5 feet tall. I used very light insect cover (Agribon AG-15) to cover the structures, connecting it to the ropes with metal paper clips bought in Staples. Comparing to traditional row cover it allows you easier access form any side, clean fabric, more air for the plants to prevent rotting. All my 25 plants already either harvested or ready to be harvested. No maggots, no caterpillars. In the same time I lost whole crop of turnips due to maggots. Turnips going under cover next year as well.