We just updated the Addon Manager this weekend, which now includes an option to turn off Ground Traffic in FSDT sceneries. It only works for dynamic traffic.
The static ones are removed automatically by GSX when you select one of its services, but this only works for the more recent sceneries ( KDFW, KLAX, CYVR, JFK V2 ) that have been designed to be interfaced with GSX. In older sceneries, you should act on the Scenery Complexity slider to turn off static vehicles that details parkings.
