When members leave our guild, I simply delete their characters and then their accounts. This removes them from just about everything, including the Raid Availability lists.
However, I believe the "preferred" method for people who leave the guild is to simply Disable the account rather than delete it. This is a good thing to do if the person may come back to the guild at a later date, but I'm unsure if it would remove them from Raid Availability...seems like it should.
As far as I know, there is no way for an admin to manually remove a character from Raid Availability.
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Ieyasu - Organizer,
Ex Cineribus