Dear Florian,
please take this path:
Administration > System Check > Overview > Ownerless Objects.
There is a list of ownerless objects. Admittedly this screen would benefit from multi-actions to batch delete.
What I would recommend is to delete all objects of a user before you delete the user. In Administration > User Management > USER NAME > Repository objects you have a complete list of all objects of a certain user.
regards
Alexandra
PS: Based on what legal requirement should a forum and blog posts be deleted after being anonymized?