How can I turn off animals in creative mode?

You can use the WorldGuard mob-spawning flag:

/region flag __global__ mob-spawning deny

_{Using: WorldEdit and MV 2.5 b641}

It turns out I was not playing on peaceful mode, when you do that, all the animals go away.

The following command stops spawning naturally:

/doMobSpawning false

Set the game rule doMobSpawning to false.

/gamerule doMobSpawning false

You don't even need WorldEdit: just type /gamerule doMobSpawning false, and then type kill @e to kill everything that's alive. Once you respawn there will be no animals or mobs left.