0 values tend not to work properly.
For NPC respawn multiplier, you can increase it to 10, or higher in the file settings, but this will only prevent respawn for a long time (not infinitely), not prevent the original spawn 
→ Let’s say the respawn time is 10 years long, this means NPCs will only respawn 10 years after being killed.
→ However they must spawn at first, when the server starts, so every server restart makes the NPCs spawn.
→ Even worse, in single-player mode, things are loaded by area, and thus things will respawn if you leave and re-enter render range (or something similar)
As Taemien said, you would need a mod to prevent spawns altogether, and while I don’t know if there is a mod like this I know exactly how to do one, but I’m not going to bother downloading the 240GB DevKit again 