The only sure way is to use the micro netting which is designed for this purpose, and to either make a 'cage' around the plants or drape the netting over them. This is both expensive and tedious.
The only other method is to go out regularly and hand pick the caterpillars as soon as they emerge and crush the little patches of eggs which you will find under the leaves.
As long as you keep the infestation under reasonable control though, Brussels are a long season crop and usually grow back from even a lot of damage to produce their winter crop when there are no caterpillars or cabbage white butterflies around. You can probably live with the pest in a way you can't when it comes to cabbages, calabrese and other summer greens.

Thanks for that, I have used a system of wire hoops and netting on other seedlings, and might do this for my second planting while they are small.