Does the protective aura of the astral deva prevent it from being dispelled?
Well, since the Protective Aura works like a mobile Lesser Globe of Invulnerability I would say yes, the Dispel Magic spell does not work into the aura and so the Deva cannot be unsummoned in this way.
Greater Dispel Magic, Mage's Disjunction, Banishment and Dismissal however do still work, being of higher level.
Technically, a Lesser Globe of Invulnerability Spell or Spell-like ability could be dispelled by a targeted Dispel Magic. Please note however that, being a Supernatural ability and not a Spell-like Ability, the Protective Aura cannot be dispelled by a targeted Dispel Magic (the description of this ability in 3.5 specifically said that it could be dispelled but the Angel could resume it as a free action on its next turn; in Pathfinder the sentence above is not included anymore, so by RAW the Aura cannot be dispelled at all, since a Supernatural ability cannot be dispelled or counterspelled unless the ability itself specifically says so).
If your GM would allow the aura to be dispelled, however, another caster could unsummon the Deva with another Dispel Magic - provided it would be cast before the Deva's next turn (when he could resume the aura again).
Just my 2c.