If you need to reset the database you can use ch_advent_reset from console.
This can be done in-game or via your server console panel.
Running the command (in this example in-game) will prompt you to confirm the action first:

Running it again will then erase all data from ch_advent_players and ch_advent_history tables in your database.

If there are anyone online it will immediately create a new fresh save in the database for them.