1. Open you Cpanel
2. Find Databases and select phpMyAdmin
3. Then you need to find your phpbb database
4. Then on left hand list select phpbb_topics
5. Click on Search
6. Find topic_last_poster_id and place a 0 (number zero) in value box and press Enter key
You will now see a list of all the guest topics found with search
7. Click on the X to delete each of them.