1. Grow them even earlier by sowing indoors and planting out, so with luck you might just miss the moth. In theory the moths don't fly and lay eggs until June so by cropping them before then you should miss it.
2. Don't grow peas in the same place for at least two years, as they pupate in the soil. Though the moths will travel a long way to find the peas of course.
3. Try a late crop, sowing in August to harvest in October or November. This should avoid the worst of the caterpillars.
4. Grow them under fine mesh (which works well, and is reusable so not expensive). You can also cover them with fleece on a light frame, but this doesn't last so long.
5 Grow them in a polytunnel or greenhouse where they are protected.
6. Spray with an unpleasant chemical if you can still buy it. Most of the 'nasties' are now banned and who wants to eat chemicals anyway. Maggots in the peas are tastier and less harmful!
7. Grow mangetout peas like Roi de Carouby which you harvest while the pods are flat with the peas unformed. You get the pea taste without the maggots. The Sugar Snap type are left until the peas have formed, though you could pick these earlier as pods too.

I think growing under mesh is the only 100% solution, as pests seem to have an unbelievable knack in finding their target plant, no matter how you try to disguise them with companion planting. I finally got round to clearing four feet high weeds around the lily plants in the overgrown flower garden and still found them infested with lily beetle.