User blog comment:Xilinoc/Ch. 570 (I can't think of a witty title at 11 pm)/@comment-24572618-20140226225829/@comment-5398287-20140302015055

"It was actually the Old man that stopped training him further seeing his prowess and realizing that if he continued, it could become a problem if he went rogue."

Actually, it was Central 46 that made Yamamoto stop the training for fear of how powerful Kenpachi could become. Yama was cool with it, but Central 46 is never cool with anything cool.