The easy way to do this is to create an exclude BGL file, excluding Library Objects and Generic Buildings (and effects if there are any flashing red lights around).
I use ExcBuilder2 for this, it's the easiest. if you are using ADE to create an SD airport you can use that instead if desired (the button with the yellow square on it). For ExcBuilder2 you will also need FSConnect installed if you don't already.
To use ExcBuilder I do this:
1. Go into Slew mode in FS (Y)
2. Go into Top Down view (Ctrl-S)
3. Press the Space Bar to point true north.
4. Slew to the upper left of your exclude area. Click the two buttons in the upper left of ExcBuilder
5. Slew to the lower right of your exclude area. Click the two buttons in the lower middle of ExcBuilder.
6. Select to exclude Generic Buildings and Library Objects (for example), or select All Objects to get rid of the autogen trees and buildings.
7. Click the Generate XML button.
8. Click the Generate Exclude File button. I give mine names that start with x (xklax_terminal.bgl).
Move the resulting .bgl file from your ExcBuilder folder to your scenery's folder, or to the Addon Scenery/scenery folder, or any other active scenery folder above the default layers.
Hope this helps,