Why do the resurrected bodies disappear in the impure world resurrection jutsu? Some of the resurrected people like Sai's brother were freed from the jutsu although Kabuto told Tobi that the resurrected people can't be killed. This is a pic of a conversation between Kabuto and Tobi.



Similarly, Sasori also disappeared after telling Kankuro to use his puppet parents. 



How can the souls be freed from the bodies even if the user who resurrected them did not release them? And why didn't this happen when the third hokage was fighting his predecessors during the chunin exams?
The souls can be freed from the Impure World Reincarnation if they are given closure. Experiencing a emotional change allows them to break out of the jutsu and the soul can ascend.  

Orochimaru stated that Hashirama Senju could break the summoning jutsu if he wanted. I'm not sure why he chose not to during the fight with Hiruzen Sarutobi.