What makes a physicist qualified to make pronouncements about theology?
A physicist is qualified to make pronouncements on what is likely to exist (or not exist) in the universe/s.
If Hawking tells me something about black holes, or the curvature of spacetime, or the probabilities of the Higgs-Boson being found next year, I will pay close attention and presume that his qualifications are beyond reproach, and above nearly anyone alive.
But his unquestioned genius for physics doesn't make him uniquely qualified to speak about spiritual and theological matters, anymore than it makes him uniquely qualified to make definitive pronouncements about Shakespeare's motivations in composing one of his sonnets. That simply isn't Hawking's area of training and expertise.
He is entitled to his theological, or atheological, opinions, as is anyone; but he is no more qualified to state that there absolutely is no afterlife than I would be to state that there will absolutely be a contraction of the universe followed by a Big Crunch.
Heaven is not a theological or spiritual proposition: it is claimed to be a
location, which presupposes that in some corner of some universe, it
actually exists. I would not go to a theologian to answer the question of whether a proposed
location is or isn't possible within the natural laws of the multi-verse as we currently understand them: I would go to a physicist. A theologian is of no use to me, except as an expert in the history and practice of religious belief.
For the record, a physicist will not necessarily tell you heaven is impossible. Since M-Theory (very fashionable) predicts an infinite number of infinite universes, many physicists who subscribe to this model will swear up and down that this means every conceivable universe not only CAN exist, but DOES exist. (I disagree with them, though - like religious folks, they tend to obfuscate the line between imagination and reality - I will confidently claim that there is
no universe where a thousand monkeys banging away on typewriters have inadvertently produced the complete works of Shakespeare just as I confidently as I will claim there is no heaven).