Awkward and Caelin are correct.
Unless a Guild Master is hacked/compromised and the issues arise from that, Blizzard won’t step in.
The philosophy is that the guild belongs to the Guild Master. They are the ones who set up ranks, bank permissions, etc. If they don’t want someone in the bank, don’t give them access, or limit it. If they don’t want someone to be able to dethrone them later, they need to set those people below rank 3.
They have a Spotlight article on it to help folks
And the Guild Dethrone article
If you were inactive for longer than 90 days and another guild member took the leadership of your guild, talk to the new Guild Master. If the new Guild Master doesn’t want to give you the leadership back, Customer Support will not intervene
EDIT - there is one tiny exception to this. If you were original GM, were dethroned, are still in the guild, AND the new GM is inactive. Sometimes they will give you the guild back. It is rare those conditions are met though.