Hi Alberto,
To clean the database you have to delete the whole records in the DB. The module account_cancel is there in the Odoo by default, no need to download it from the Odoo apps.
You just can search account_cancel in the apps, so that you can find it. Once the module is installed it will help you to cancel the journal entries . There is some community module for database clean up , unfortunately its v10 is not available, https://www.odoo.com/apps/modules/9.0/database_cleanup/
As your testing is completed you can drop this database and you can create a new db for the rest of the operations, that will be better.
Thanks
Thank you very much Niyas. I'll follow your suggestions!