Dean –
Get FTP access and then delete the /wp-content/plug-ins/tweet-old-posts directory. FTP access is a bit complicated. You will need a good FTP program like Filezilla and then you add the FTP site to it. Filezilla is free. Download and install it. Put the IP, username and pw into the program and if you get logged in your website will appear on the right half of the app. drill down in to the /wp-content/plug-ins directory and then right click on the tweet-old-posts directory and click REMOVE. Be careful with FTP. You can delete good stuff with it.
Stu