Prevent server from saving Worlds

How can I prevent my game from saving a World?
I tried:

PHP:

$this->getServer()->loadLevel("arena");
$this->getServer()->getLevelByName("arena")->setAutoSave(false);

I unloaded the level and Loaded it again, but the changes got saved.

PHP:

$this->getServer()->unloadLevel("arena");
$this->getServer()->loadLevel("arena");