Fine netting will keep them at bay or you could rely on hand picking on a daily basis if you only have a few plants and have time or the inclination.

Dianebulley told me to grow some Moschata alba with them. I have two plants of it now and the butterflies do not land on the brassicas at all even though they fly around them.

Don't use netting - it's bad for birds as they can get tangled up in it and die a horrible death.

If you want to use physical protection, use chicken wire with a small hole size (Screwfix sell it for a fraction of the price charged by garden centres). We make cages with it using plastic ties, to keep both birds and butterflies off vulnerable veg, and it works well. :)

Thats a good idea -have you tried making fruit cages with it?