You can assign an ecosystem element to each shade of gray, but suppose you want one shade clear? I want my roads to be clear of huts. I'm experimenting, just using black for the hut zone and white for the roads and concrete. The first object loaded in the ecosystem is assigned to the darkest gray. The second is assigned to the next shade up. In this case, I loaded and deleted an object for the roads and then populated the ecosystem, but huts are intruding on the roads. 

Maybe it's the sampling level. I'll raise it and see if things improve. 

Am I on the right track with my attempt to clear a gray zone of all ecosystem elements?

Download my free stuff here: http://www.renderosity.com/homepage.php?page=2&userid=323368