Blizzard added a couple of fixes to the known issues list, and both remove Diablo 3 EXP exploits achieved by spawning endless waves of enemies. Here are the two new fixes:
The Demonic Prisoner event was the main exploit though. That’s an event that spawns in the Battlefields of Eternity area of Act Five, and has a Unique boss named Emikdeva that you need to kill to end the event. Until he dies, his minions keep spawning from the portal, and they’re worth good exp. So of course players figured to lead the purple far away and park him, or use the waypoint to get back past him, and then kill the minions forever. ETERNAL BATTLEFIELDS = ETERNAL EXPERIENCE! Allegedly adding up to around 1 billion per hour on T6.
And I guess you can still do that, but there’s no point since the exp won’t continue to flow. Details about that seen in a forum post here, with the video tutorial from PaulHalTV.