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.

The tools i mentioned are those:

I've tried everything I know possible. Which tools would you recommend? 

I definitely used the correct method of deletion as I've done it countless times before (Simply deleted the member, then clicked "Delete all WpForo content") however since there were thousands of posts to delete I suspect it got overloaded and crashed midway through deletion which resulted in the posts still being there but no member/author. 

Apparently you didn't deleted correctly (when manually).

It was far too many to delete manually so I entered the database and deleted them all from there. 

However, the posts are still visible from the front-end, sort of. They show up but visiting them will show an empty post. Is there a way to remove these posts from being visible front end? 
I really appreciate any help possible!