option 1.)
players don't get a +1 stats kill by helikilling but they won't get punished for it either.
This might decrease the amount of heli's.
Option 2.)
Punish helikillers.
if it's a real problem & you really want to decrease the amount of helipilots helikilling, explosions when the heli pilots crash & the random (new) players get pissed off & quit.
This probably will decrease the amount of heli's even more than option 1.
Option 3.)
Leave it as it is.
I don't really get mind getting chopped by a heli,sometimes it even turns me on
others do get mad & quit, but don't they get mad @ other stuff aswell???
Option 4.)
If the real problem is actually about the "nam wars", then why you don't create a "anti-flying zone" around that area?
So that when people fly into it with a heli or plane they must get out of that zone as quickly as possible or get killed.
Option 5)
take the nam wars somewhere else where helis can't get by