When you don't select "enable castbar" for you player frame AGUF shows the default blizzard castbar. I am using ecastingbar as a castbar replacement.
Right now I don't use the aguf castbar but I do use ecastingbar. This combination leads to the blizzard castbar also showing. Could you add an option to hide the blizzcastbar even though you are not using the aguf castbar?
Make sure you're reloading/relogging after fiddling with those types of settings, unchecking a casting bar in most addons will tell it to unsupress the blizzard bar, while the code that tells the addons to supress it generally isnt run after login, so it will be supressed after you relog, but not until because you specifically told it to unsupress via unchecking it. That is how it works in most cases, anyway.