How can one undo sorcery? Does it dissipate with the death of the sorcerer or the death of the one who sought the sorcery? Are there ways to undo sorcery without resorting to a raqi?

There is nothing that definitively states that the death of the sorcerer or the one who requested the magic breaks the magic spell. Magic can be broken by performing ruqyah in accordance with Sharia, which the Prophet, peace and blessings be upon him, used to perform upon himself, and it is more conducive to sincerity. It is also permissible to use some lawful and proven things such as Sidr (lote tree leaves), by crushing seven green leaves of Sidr, mixing them with water, and reciting certain verses over them, such as Ayat al-Kursy, the Mu'awwidhatayn (Surahs Al-Falaq and An-Nas), and the verses of magic from Surahs Yunus, Al-A'raf, and Taha. The afflicted person then drinks from it and washes with the remainder. This has been tried and is beneficial, by the will of Allah.