Spoilers on Kizumonogatari below:
In Kizumonogatari, Oshino Meme says there is a way to keep Shinobu alive that would also stop her from eating humans. He says Araragi must almost kill Shinobu to do so, which turns her into a quasi vampire being. From what I understand that is how Shinobu lost most of her powers.
However, how did Araragi lost his vampire powers? He had just sucked Shinobu's blood a little earlier which made him more powerful and I don't see how almost killing Shinobu would make him lose his powers.
Also, why did Araragi stop wanting to eat humans?