As Ray pointed out, there's no easy way to 'clean' a database. It's possible to delete/reverse some transactions, but that may have subtle effects that do not show themselves until later down the line.
The best advice is to set up a 'baseline' database, which contains configuration/settings, but no transaction data. This would ensure that you always have a 'clean' state to revert to should something go wrong with your live database. I'd also suggest configuring daily backups, as this saves a LOT of headache should something go wrong.
Backups module: https://apps.openerp.com/apps/modules/8.0/auto_backup/