You can't get rid of "baked in" vegetation. If you are using the full warband scene editing tools, there is a step where you initially create your terrain, and if you want full control over vegetation, you can indeed set the respective slider to 0, then place all vegetation by hand (which is a major PITA, though, mind you). You will still have grass on the default grass terrain texture, but this can be removed by painting the terrain with a different texture.
If you are using the quick and dirty way of editing, you are out of luck. Your only chance would be to play around with terrain heights a bit more, as I've noticed that, especially if you lower some terrain below sea level, playing around with terrain heights can also redistribute vegetation.
