In Zombieland (the one from official download) you can bomb beyond the territory PU value. This means zombies cannot collect income. This is the ONLY way humans can win since zombies have the advantage.
Whereas in Zombieland USA (fro ROgerCooper site), they removed bombing all together and introduced a lot of unnecessary things. This makes the game too complicated and the battle calculator cannot handle all the complicated rules.
Could there be a way to lose the zombie production and instead make them convert dead US units into zombies?
You could have a ZombieCreator unit that products a zombie each turn. Make it capturable. If taken by the humans it could become an InactiveZombieCreator unit. Make the unit invisible and it should behave the way you want.
TripleA does not support any triggers involving destroyed units.