From server.properties

Log In to the Control Panel ↗

Select Your Server

Stop the Server

Click on the Files tab

Open the server.properties file

Set the Spawn Monsters Value

Find spawn-monsters=[value]. Change [value] to false to disable mob spawning, or true to enable it. Click Save Content.

Start the Server

Gamerule Commands

Using gamerules is the recommended way of controlling mob spawning, as it gives you more granular control.

Run these commands in-game (remove the leading / if running from console):

Toggle General Mob Spawning

/gamerule doMobSpawning false
/gamerule doMobSpawning true

Toggle Phantom Spawning

/gamerule doInsomnia false
/gamerule doInsomnia true

Toggle Pillager Patrol Spawning

/gamerule doPatrolSpawning false
/gamerule doPatrolSpawning true