Spawn killing isn't really the problem, it's the OPness of humans. If the god and titan kits are nerfed and OP zombies re-added as I suggest then spawn killing will rarely be possible.

 

All donor kits will probably be nerfed/removed in the future. Humans would still be spawn killing if donor kits were to be nerfed, and is why I love @TheEnderSlime 's idea of giving the zombies a few seconds worth of invincibility so they can escape the spawn point.