Hi

There are several ways to deal with this - the best IMO is to download ACMan.exe (an Aircraft Folder Manager - set up any number of hangers with a/c specific to theatre/active date) or the more recent Theatre Changing Utility by cliffxxx (allows you to set up theatre specific a/c and missions). 

Another way is to edit each a/c's .xdp file (with say Notepad) - find attribute "AllowSpawn" and change "Value" "y" to "n".  Then it cannot randomly spawn in any circumstance - less flexible (?MP compatibility). 

Capt Black