My mod currently doesn't support deleting resources and I don't plan on adding that at the moment. (You can read more about the complications with that in the Design Choices thread if you're interested.) You can however install Ore Eraser for this, or open your safe file in the map editor, where you can also delete trees and change tiles. And if this happens again, you can also undo the placement by pressing CTRL+Z. This only works immediately after placing resources though, so it's probably too late for that this time.
