Why doesn't Odin remove Hela's powers?
In Thor, Odin was able to remove Thor's powers with an incantation.
Why didn't he do this with Hela, or for that matter Loki in the second film?
Odin didn't remove Thor's power. He removed Thor's ability to channel his power through Mjolnir.
In the original Thor, Odin simply removes the hammer from Thor's hand. He then puts an enchantment on it that only the "worthy" can wield the hammer. Thor does not know that he can use his power without the hammer as evident his conversation with Odin in Thor: Ragnorak.
Thor: She's too strong. Without my hammer, I can't....
Odin: Are you Thor, the God of Hammers? That hammer was to help you control your power, to focus it. It was never your source of strength.
As such, there does not seem to be any evidence that Odin can actually remove innate power. It simply would not work on Loki or Hela as they both seem to have figured out how to use their powers without a tool.
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How was Odin able to keep Hela away?Unable to kill his own daughter, Odin defeated Hela and imprisoned her using his own power to keep her at bay in another realm for eons. During this respite, Hela attempted to escape imprisonment in Hel and was near successful. Odin dispatched the female warriors known as the Valkyrie to stop her.
When did Odin banish Hela?Hela was defeated in a battle that claimed all but one of the Valkyrie's lives and was locked away in an interdimensional prison in Hel, with Odin's life keeping her at bay. Once Odin dies, she breaks free and thwarts an attack by Thor and Loki, during which she destroys Mjolnir and banishes her brothers to Sakaar.
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More answers regarding why doesn't Odin remove Hela's powers?
Odin,. Thor and Hela all draw their strength from Asgard. That's why Odin couldn't take hela's power because he lost his power when Loki took him to earth. He had banished her but by him not being on Asgard anymore her banishment wore off and she escaped her exile.