I think WAMP has already a utility called phpmyadmin(mysql manager from where you can create databases, and manage existing ones) that will help you with this, try accessing the
http://localhost default page that wamp provides and from there click on the phpmyadmin link!
Also to empty a database, you need to select it in the phpmyadmin and then delete all the tables from it!
If you still can't manage it this way, JoomlaCloner has an option that will help you with this, in order to use it, you need to re-generate a backup on the original site, and click in the "Database Options" tab the option "ADD DROP syntax", before you start generating it! This way, when importing, the JoomlaCloner imported will delete the existing tables from your database and import the new ones!
Ovidiu