1. Here's how

When you place a model on a flat terrain and turn on Landscape Grass, it sometimes looks like this:

1.1: To remove the Landscape Grass on the floors and patio, simply select a different, unused Landscape Texture Slot than the one that Landscape Grass is assigned to:

1.2: Then select the same grass Landscape Texture which is used for the Landscape Texture slot that the Landscape Grass is assigned to:

1.3: Paint the areas to make the Landscape Grass disappear: