This plugin stops certain mobs (you can add and remove them in the config file) from spawning. The goal of this plugin is to remove mobs the player doesn't want in order to make the game more fun/comfortable.
When installed, the plugin creates a folder in the plugins folder called MobRemoval. So your file structure should look like <folder containing server jar>/plugins/MobRemoval
Inside of that last folder should be a config file called config.yml. Inside this file you will encounter 2 entries: isEnabled and disabledEntities.
IsEnabled is not important for now, disabledEntities is the one you want to edit. Follow the instructions in the file to add or remove entities.
In-game you can use /spawnblocker <true|false> (replace it with true or false) to toggle the spawnblocker, by default it's enabled. Using the /spawnblocker command without specifying true or false returns the current value, so you can use that to see if it's enabled or not. Using /spawnblocker <true|false> has the same effect as going into the config file and setting the isEnabled entry to true or false.