There's currently no means of restricting toolpath generation other than with the canvas' volume. You can use a raster-layer to mask off areas of the canvas so that generating a toolpath only occurs in the un-masked area. Create a paths-layer and draw a path around the area you want to mask off, then use the Shapes From Paths function to create a flat shape. Invert the resulting raster-layer and change it's background Z to the top of the project, so you have something like this:
