Is that even possible, now? I thought they simply despawn if you release them... If not, I've just found my new favorite past-time .
I just had an interesting conversation with a fellow Warlock about Doomguards and really bad PUGs. He says he will summon one and release it after a wipe and let it stomp all over the PUG.
There are three secrets to playing a great Warlock. First, ... Second, ... And finally, never to reveal these secrets to anyone!