There’s a self-service refund section on the Support site. If it’s unable to be done through self-service, then there’s an option at the bottom to open a ticket for a manual review.
Support home —> click World of Warcraft under the Games heading —> scroll down past Article list and click Payment —> scroll down a bit and click Get Help button —> click Refunds —> click Refund Character Service, Guild Service, or WoW Token option —> click blue Continue button