To delete your portfolio, log in to your account.
If you are a Premium user and wish to cancel your subscription but remain on the free tier (with restricted access):
1) Click 'Billing' at in the navigation bar at the top of the screen
2) Click the 'Cancel Premium' tab
3) Follow the steps through to confirm your cancellation.
If you wish to cancel your account entirely and remove your portfolio:
1) Enter your account settings by clicking the cog at the top right of the screen. 
2) Click the 'Delete Portfolio' tab
3) Click 'Permanently Delete My Account'
If you a Premium user, this will immediately cancel your contract with