[Simon Lloyd]

Make sure you have a database backup before running this as i'm no database guru, in phpmyadmin run this:
delete from user where username='imported_%';

You will have to add the prefix if you use one in the tables like vb_user and vb_username, the % is a wildcard so it's looking to delete anything in the username column that has imported_ at the beginning of the name.

Hope it helps.