The Hollywood Reporter spoke exclusively to Zimmer on the matter, and he was pretty blunt in clearing up any association he may have with composing the score to Zack Snyder's Superman. Below is an excerpt from their interview..

THR: The internet is buzzing with rumors that you’ve signed on to Zack Snyder’s Superman project. Any truth to that?
Hans Zimmer: You know, it’s like some dirty quote taken out of context. Let’s just be absolutely straight here: I have never in my life met Zack Snyder. I think I need to give him a ring...
..Zimmer: I met Chris Nolan once and he knows Zack Snyder and therefore I’m doing Superman. You know that’s all it is.
THR: It may stem from the fact a lot of people would like to see you take that project. Based on the Batman films.
Zimmer: How can I say it: My heart belongs to Batman. I wouldn’t even know how to go and give voice to it. I haven’t thought about it.
THR: Not to mention following in the footsteps of John Williams.
Zimmer: Right! John Williams, the greatest living composer -- full stop. And that happens to be one of his greatest themes. So no. And I’m not thinking of rewriting Beethoven’s ninth either. It just sounds like a thankless task, you know? So that’s unequivocally a no. I have never spoken with Zack Snyder.
That sounds pretty definite to me. Its a shame though. I understand him not wanting to mess about with Williams' iconic score, but I think he would have made some great ear candy for Superman to fly to. Lets hope he does pick up that phone.