user with banned are just change the role in main site from "user" into "banned" but the forum will be fine.
thing is when you receive banned role they disable almost every permission on the site while the site also trying to call those function but the api return access denied for that due to the role. (the function such as "listing the read titles, the manga list, user profile edit" and some other so basically it just let you login with the power of guest member like you didn't login at all.)
And they also disable the permission to request account deletion