Theres probably some direction functions you could hook and setup as a forward to block. Alternatively, doing it all in sourcemod, you could just cycle through the entities and destroy the ones you dont want if they exist. I believe that even the NPC zombie bosses spawning call like they were actual players so, you could probably hook these game events instead "tank_spawn" and "witch_spawn" and just destroy the entities of "tankid" and "witchid" inside of them.