I think I can answer this one, actually, as there is a metaphysical explanation for it - Heaven would be the concept of "ultimate positivity" - a subjective, individualized experience that induces feelings of perfect comfort and happiness. There is no pizza or sex or even "fun" in worldly sense; there is just an everlasting feeling of complete contentment.
This poses problems, however, as everlasting contentment and the complete lack of desire for anything else technically qualifies as sloth, a sin in Christianity.
It would not, however, constitute the end of free will; the institution of absolute perfection, however, would be unresistable. The perfection of that happiness would preclude all things that might cause a desire to leave; such as the knowledge that friends and family are still alive (and not happy). The desire to return would be quashed somehow, although not necessarily forcibly; anyone who has not consciously decided to reject heaven purely because of what it is can be convinced to stay. I'm going to go out on a theological limb and say that no one who would consciously reject heaven would ever be let in to begin with.
So that's heaven - the concept of perfection to the human mind (or the human soul, take your pick). You don't get in if you're predisposed to reject the concept of it. If you do get in, any desire to leave is quashed by the perfection of the place, making the desire to leave unfathomable, even for the best of reasons. The trick isn't to remove free will - only to make all other choices look completely idiotic.