Board Thread:Questions and Answers/@comment-4556003-20130502152512/@comment-2211792-20130503192231

He might have been able to kill Aizen even without Hollowfication, had he pierced his head instead of his chest. And then used his Kill, Kamishini no Yari power. I'd be very surprised if Hogyoku could regrow Aizen's head.

Otherwise, no; not even Hollowfied Ichigo could do much to harm Aizen even before he began transforming.