I have a similar problem in Fury Road, but I used a different solution. In my mod, I treat the hero units as being "very experienced". So they have to get a lot of kills before they get any more promotions. This may or may not be what you want. Do you want the hero to get another promotion with only 2 more kills? Or more like 15 kills?
What I did was start the hero unit with 15 XP and level 4, with a couple of lines of python. So a GG can affect this unit, but only gives one more level. That makes it much less popular.