There may be other methods to do this in C5Pro; however, the only way I know how to do this is by using the AnythingGrooves plugin from Digital Carvers Guild http://www.digitalcarversguild.com/plugin.php?ProductId=4

The "Zero is empty" option will not create vertices/polygons where the map is black. You would use a map with a solid color where you want to keep polys and black where you wanted to remove them. Varying the solid color (darker is better but you still may need to see it for reference) and the height values should allow you to remove the areas in black without displacing the other areas.

I'll do something later today to show this.

Chris