One problem is with some of the maps - infected_hospital, castle_age, to some extent feartowr, where you can physically enter the spawn and kill within it.
Of course the other component is the complacency or acceptance of said practices, like there honestly is no problem with going into spawn and doing that - as opposed to being outside of the spawn area and killing as they come out or something like that.
One thing is that the green fog heals you incrementally, and *IF I REMEMBER CORRECTLY* only in the presence of 1 or more other zombies? THAT alone lets you get killed by higher power weapons, or by large numbers of people, very quickly when in spawn. One possible solution could be therefore that when 1+ other zombies are present, the green fog fully heals zombies standing in it instantly - instead of incrementally. Of course, that has its own problems associated with it I guess. Another thing is, don't zombies recoil from being shot? As in, for a split moment they can't attack, use special abilities, or even move? If so, looking at modifying that may be a good idea.