Make a new named-layer in your landscape material called “NoGrassPainter” (or whatever).
Subtract this from your grass-maps, multiply, lerp2zero; whatever you math to get the grass-layer to zero. You can paint directly in your landscape to paint-out grass.
In your spline, you get a reference to your landscape and ApplySpline to paint that layer in for you, wherever the spline is it will paint NoGrassPainter along the way.
If done properly, it ought to paint out grass.