Bukkit

This is the default MultiWorld Config File :

But you can enable or disable worlds with commands, take a look here

#This is the LavaBoat+ config file for worlds.

#Define your worlds in which LavaBoat+ is enabled below:

world=true

AnotherWorldNameHere=true

For add new worlds type the next Line NameOfYourWorld = true if you want LavaBoat+ will be enabled in this world and change the true into false for disable LavaBoat+ in this world.

After editing this file you have to reload your server or restart it.