Anything can show up or spawn in a campaign or mission as long as it has a y (yes) in the allow spawn, and the day of the mission is within the entered service date and left service date. Open you xdp file and scroll across top line to see what those are. If you choose that the particular item is one you don't want to randomly spawn, change the allow spawn from a y to a n (no) and click open file and click save. Changing the xdp will make that item unavailable for multiplayer.
Some add on aircraft have incorrect entered service dates. The beta He-162 has a 8-8-44 entered service

and it actully became operational in March of 45. You can change the entered service date so it doesn't show up over D-Day. Some of the add on P-40s have the date it entered US service. AvHistory has their P-40 enter at 1-1-42 8)so it reflects when it would have been in combat. Others don't and the plane will show up in BoB where it might not be wanted

. Change the entered service date or the allow spawn to n and problem solved.

Wulfmann