I’ve had to do this a few times when I wasn’t able to just drop the entire database, so I threw together a small PHP script that will do it for you.
Just add your username, password, and database name and save it as drop_wp_tables.php. If you have shell access you can run php drop_wp_tables.php. Otherwise upload it to your site and pull up http://yoursite.com/wp_delete.php. Make sure you delete this file if you upload it to your site!
Note: If you used something other than wp_ as your table prefix, make sure to edit the script accordingly.