The editor will readd that line every time you save, meaning the best course is to separate the objects into a separate file which editor will leave alone.
However, if you need to edit the placement of these objects, you'll need to add them back in or load them as staticobjects.con but have no reference to where other objects on the map are and so on.
There is another way which also has its ups and downs but I believe is more practical.
Requirements
- BF2 Editor
- Notepad++
Next, run Notepad++, go Plugins>Plugins Admin and install ToolBucket plugin.
Open your map's staticobjects.con file in Notepad++, go Plugins>ToolBucket>Multiline find and replace.
Find an object in your new layer, copy the layer and isOvergrowth line, change 1 to 0, Replace All.
All objects in that layer are now isOvergrowth 0 and will not switch to _lod while any other overgrowth object in static objects that should switch, will remain so.
The downside is that editor will undo this when you again save static objects, so repeat this after each save or when you're done with editing static objects.
The upside is that you do not need to store those objects in a separate file and can work with those objects much easier as everything is in one file.