I am not sure what you mean by leaf cutter. There is a leaf cutter bee that removes section of leaf mostly on roses that causes no harm. On Brussels sprouts and other brassicas it is usually a cabbage worm making large holes in leaves. Cabbage worms are small green butterfly larvae that hide under the foliage but leave droppings on the leaves. To prevent them you can cover the young plants with agricultural fleece or row covers to prevent the white butterflies from laying eggs on the plants that will then hatch the green worms. Handpicking the worms in the early  morning may be the most practical option at this time. Keep Growing, Marianne Binetti