I did a bunch of digging and wasn't able to find help on this issue either when my AT&T 6 preorder shipped long after I canceled. Know it's AT&T rather than Apple, but...
I had UPS hold it at the local branch and went in and told them it needed to be returned/refused. It was delivered back to AT&T a couple days later, and my eligibility was reinstated the same day it was delivered.
My returns to Apple through their normal process have usually been pretty quick and painless, though.
Again, AT&T rather than Apple, but just sharing UPS "refuse delivery" experience...