How to delete all the statistics for a given websites, or for all websites?
It is possible to keep all the websites information (names, defined goals, etc.) and only delete the stored analytics data. However, there is currently no tool available to do this: you can directly delete the data from the Piwik database. Do the following:
* Delete all the mysql tables with a name starting with piwik_archive_* ; they will automatically be re-created when Piwik processes the reports
* Execute the following query: DELETE FROM piwik_log_visit WHERE idsite = X with X replaced by the website ID you wish to reset.