Ok then, wait until someone from wpForo support will help you out of this.

@nylex,

Copy the post content or part of it, the go-to Dashboard > Forums > Moderation > Published, paste the copied content in the search box, press Search Posts button, after finding the post check it's Created By column. The column can be empty or set to anonymous user, if there is no user, you can go back to Published Page, use the Created by button to order the posts by usernames, and search for post with no username. Use Screen Option to set the count of items per page.

If the username is set to anonymous, go to Dashboard > Forums > Members, find the user with the Search box, press edit profile button, In the profile page press Dashboard Action > delete in Dashboard.