is there a way to let members delete their account and automatically assign his posts and threads to a defined user called "deleted" for example?
is there a way to let members delete their account
Please note, wpForo is a forum plugin, it's not Profile Builder or Social Community Manager. It doesn't have such feature. There is no way to delete own account. You should search for some other plugin for this purpose.
Info on how to delete members is described here: https://wpforo.com/docs/root/members/member-manager/