Welcome to Heavenly Host Elementary School! But...
This is simply a mod that allows you to stop mob from natural spawning in a specific region, which can protect dark buildings and so on.
The type of mob can be specified, e.g. monster, creature, ambient...
It is rather feature-incomplete now, but it is usable.
Usage: /peace
Executing command without arguments to see full help.
Note: it is possible to do so in vanilla, by stacking monsters. However it is player-centered: if you have 70 monsters in your r=128 range then YOU will not spawn monsters in your r=128 range, thus disabling spawning in a region would be a sphere-covering problem, which is very complex and increases MSPT much. This mod is entity-centered and is very easy to understand and control.
