How to prevent landscape "grass" nodes from spawning meshes along a spline road?

I have a Landscape with a Grass Output node. When I run a spline on top of the Landscape, and attach a road mesh to the spline’s segments, the grass appears through the road.

How can I disable this behavior? I’d like either the road mesh or an area to either side of the spine to block the meshes provided by the Grass Output node.

The Landscape Material doesn’t seem to have access to the road mesh or the spine, so I’m thinking this might be done by using a Blueprint to traverse the spline and apply a blocking volume of some kind as it goes. Or perhaps an exclusion layer in the landscape that could be applying by painting. However, the details elude me. Suggestions welcome.