Depending on how much work you want to do, it is possible to have categories and forums with identical names. This means that you could completely duplicate your existing category and forum structure, giving a No access role to all the duplicate categories and forums for the Registered users group. Leave them empty of topics or posts.
Then, identify the username and id being used by the troll. Give the troll a Standard access role in Users' forum permissions for all the duplicate categories and forums and a Never Can see forum User's forum permission for the actual categories and forums. Might want to disable the forum while you are switching permissions.
Now, make a global announcement In one of the duplicate forums:
Due to spam we have decided to remove all topics and posts and start over. The old topics and posts will still be visible to Guests but once you login you will no longer see them. Feel free to start new topics and posts.
Or something like that.
This also assumes you do not allow Guest posting in the original categories and forums.
The result of all this will be that your troll will see the forum as normal until he logs in, at which point all the categories and forums will be there, but empty. He is free to post away. No one will answer him and no one will see him.