Okay, I'm paraphrasing here. But there's no way you can read Quinto's unambiguous "I'm not going to do it," when asked by the Associated Press as to whether he might return to the role of Sylar for the Heroes sequel series as anything other than emphatic.
Don't believe me? You can head here and watch the interview video (about 1:05). He quickly mitigates his declaration by saying something about the timing being wrong and being grateful to the show, then mitigates his mitigation by stressing just how extremely interested he is in "new experiences" which are not Heroes. Star Trek 3, on the other had, seems for some mysterious reason to be more palatable to him.
I don't blame you, dude. Stay far away. You've served your time.