how about massive winds that randomly blow the off map guy all over creation? Maybe give them a couple min warning that there is severe weather incoming???
Orrrrr have an uber set of AI fighters swoop in after a set time and tear the offender up?
"Radar has detected heavy enemy contact in your area!"
Engine efficiency go's to crap after a short time and off map aircraft start guzzling fuel...
All kinds of fun things to do to encourage folks to stay in the play area.