It's not practical currently without mods. At most you could set the number of people to scale to, but it's unlikely to be persistent - also any client that is first to load a zone "owns" it and for all I know may not respect the setting anyway.

Difficulty options are supposed to be coming, but as of today we have no idea in what form - we might get genuine granular enforceable server options, or we might get a dumb slider, or anything in between.