Yes you can remove in F9 mode ...
First ..select F9 mode ...
Press " E" for Entity mode
Walk up to a tree for example so it highlights ..
Press LMB to select it ... Then press RMB to delete it ....
If its grass ... Stay in Terrain mode .... Make the yellow circle as small as possible using the minus key..
Then hold "shift" and "LMB" and brush over it
Hope that helps
The only person ever to get all his work done by "Friday" was Robinson Crusoe..