Error deleting Google account from Samsung Galaxy S4
In case nothing else helps, you can always factory reset the device, that's gonna do the trick for sure. Keep in mind that this will delete all of your data and everything else, too, so just use this as a last resort.

The only other thing you could try to do is to delete the data of the settings/account related apps, but that's also potentially risky, so make a backup of everything first.