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. 
