Option 1: Use spherical falloff zones and adjust them so that that area is outside of them. Option 2: Skip buttocks, go straight to thighs. Most dresses do that. Have you checked how well that works? I have better luck with it usually. Option 3: Do not assign any leg parts to the thighs or buttocks at all. Instead have the buttock or thigh parts, but have them geometryless and leave the parts you have cut as part of the hip. Then set them (the buttocks or thighs) to use spherical falloff zones to affect the hip like they were really there but without being there. If you wanted you could add little bone-looking pointers to the mesh with an invisiblised material that won't render and the element display type for them set to silhouette or something, just so you can see what you're doing, and make those the thigh or buttock parts.