Netting them is the best way, even then they will do their best to find a way in.

Get some Malva Moschata Alba seeds. It might be a bit late, but worth an experimental try. Sow now. Keep damp. When they grow and flower they will attract the hoverflies about 8.30 p.m. on summer evenings.
The larvae from the hoverflies feed on the cabbage root fly until the evenings get cold, about Sept 12th. Then just spray for one week until the hot weather is over.

Or you could wash or spray under the leaves with a special spray with inverted nozzle.

I used to stay down the allotment on summer evenings,
just sit and watch the hoverflies arrive. Wonderful insects .

Nasturtiums. I was looking for a reply I made to a similar question a while ago. I'd heard a Gardeners' Question Time programme on the subject but I can't find the thread - they were talking about disguising the outline of the cabbages which confuses the butterflies and means that they don't lay eggs.

Netting every time - but make sure it is fine mesh and goes down to the ground

companion planting of nasturtiums will help reduce the number of eggs laid on the cabbages. but if the netting is in contact with the leaf the females can still lay their eggs. check leaves every day for caterpillars and pick off and dispose of. at home they go on the bird table. the robins come down when OH walks towards the cabbages!

Diane kindly sent me a Moschata alba and it worked like magic. The butterflies arrived and flew round but never landed. Easier and cheaper than netting.