If he pops from a scareling (Totally random) and you DO NOT want to spawn the Ireblind, Just do what I (Warrior) do, get aggro and run him away from the raid. Have a chanter follow you. Tell chanter "When I stop, Mem blurr him". Arter the mem blurr go back to the raid and Enraged IMP stays put and is again indifferent.
He spawns non aggro, however he will assist any other mobs in the area. Also it seems there can only be one up at a time. We had him pop at the west wall, moved and then had him pop again. Once that happened the one at the entrance poofed. He also will wander back to his spawn point (the place you killed the scarling) when he de-aggros so you can't move him.
As stated before he can't be mezzed, or snared, but he does move fairly slow so he is easy to outrun, and he does not summon. If you are just attempting to clear, and don't need the IreBlind I'd suggest offtanking with a FD class until any adds are cleared, then camping the party. Once everyone is off his list he will become indifferent, then just move your group somewhere else.
Just to let u guys know....this guy cannot be mezzed, or rooted, or slowed. He cons indifferent when he spawns. The bard in our raid aggroed him and zoned him but he came right back and sat right at the entrance. He does assist the scarcelings and spawns the Ireblind upon death.