I am a firm beleiver in prevention is better than cure and whilst keeping the butterlies off is a great idea, putting them off is much better.
I make a barrell of rhubarb leaves steeped in water and sprinkle it over the crop on a weekley basis. This really put them off, also, get a couple of large (ish) containers and plant them up with nasturtiums, this gives a long lasting companion plant which the butterflies love.

The combination really does the trick but you have to keep up the watering. Its cheaper, and you get some lovely flowers which you can eat in a summer salad. Win Win situation.

Darren