I had the same problem, there are some fixes and work arounds like Salmo mentioned but sometimes they work sometimes they dont...
The best method I found and simplist was just block the "bad" spawns with a static aircraft or even a truck or buildings... Things that make the base a little alive. Then just let people spawn on the good ones. And a heads up, there is a massive dip in the field at Coquelles which you can only cover with the largest French hanger.
In my oppinion I would not spend to much time trying to fight the spawns, it will drive you insane and never quite work as intended but its your mission so its up to you.
On another note:
David you know the RAF fields are call RAF Kenley etc, what would the LW fields be called?