Sniper and Assassin mods are custom mods, with is mean, that they already have an re-spawn system (especially this two have). As i know this are enabled in Zombie Plague Advance not in Zombie Plague 4.3. or 5.0.
You can just remove the custom modes from the plugins-zplague.ini or i can remove the re-spawns on them. But you have to wait a little.