To completely delete all your portfolio data, simply delete all your portfolios.
To do so:
Tap the portfolio selector at the top of the portfolio screen above the portfolio holdings overview.
Once the portfolios menu is open - tap "Edit"
Then on the red delete icon (red trashcan) to completely remove a portfolio. The app will ask for your confirmation before deleting.
Please note that this action is irreversible! The portfolio cannot be recovered EVEN IF you have a recovery code because the recovery code is dynamic and it is not a snapshot.