Or did you want to avoid getting to the caterpillar stage at all? I've been round squishing eggs where I find them. They take a while to hatch, so there's a window of opportunity.
But having said that... they've still shredded my purple sprouting broccoli leaves! So I must have missed a few.
I saw a neighbour built a beautiful cage of fleece round her cabbage seedlings, and a solitary cabbage white was flapping around inside it, unable to get out. A few days later - no leaves left. ::)