Delete all is easy. Stop server and delete database/history.sql in storage directory. After start of server it will create a new empty database. Also delete content in reports directory if you have enabled pdf reports. These will not be seen otherwise.
Not really sure if a delete all button is a good idea. Normally you want to keep them and they do not really use up a lot of disk space. For the pdf reports you can limit store to the last x so you can control automatic delete of them already and they are the only part taking disk space.
