Run the command /control
Click on 'Settings'
Click on 'Command blocks'
Close the menu
Run the command /reload (This will reload your server, it usually kicks all online players.)

Run the command '/gamerule commandBlockOutput false' (Make sure your capital letters are correct!)
 

Spawner: /mincraft:give @p mob_spawner
Command block: /minecraft:give @p command_block
Barrier: /minecraft:give @p barrier

Replace <name> with the player who's head you want.

If you have essentials: /skull <name>
Example: /skull AmazinglyJava

If you do not have essentials: /minecraft:give @p skull 1 3 {SkullOwner:"<name>"}
Example: /minecraft:give @p skull 1 3 {SkullOwner:"Notch"}</name></name></name>

Run the command /control
Click 'Manage Plugins'
Install MultiverseCore (At the time or writing this, it's the first one on the second page.)
If you want a void world, also install VoidWorld
Run the command /reload (This will reload your server, it usually kicks all online players.)

If you were kicked, rejoin your server.

Replace <name> with the name you want the world to be called.
Replace <type> with the type of world you want to generate.

Run the command /mv create <name><type>

Types: (Void worlds require the plugin VoidWorld)
Default world: normal
Nether world: nether
End world: end
Super flat world: normal -t flat
Void world: normal -g VoidWorld</type></name></type></name>

Run the command /control
Click 'Manage Plugins'
Install MultiverseCore (At the time or writing this, it's the first one on the second page.)
Run the command /reload (This will reload your server, it usually kicks all online players.)

If you were kicked, rejoin your server.

Replace <name> with the name of your world.

Run the command /mv tp <name>

Default world names:
Overworld: world
Nether: world_nether
End: world_the_end</name></name>