You can delete them all in MySQL.
** Please make sure you have made a backup of your data, just in case. We are not responsible for your lost data **
In your MySQL client, issue the following:
This will clean out biblio, biblioitems and items (by cascade), leaving you with a fresh biblio database for production use.
Additionally, until you clear the indexes, your deleted items will still be searchable in the database.
Posted in: Installing/Upgrading