A user's Pico account can be deleted one of two ways: either you can delete a user or the user can delete their profile.
Select the user's record in the Pico Contacts, click the menu button (3 dots), and select Delete user.
Or the user can delete their account by logging into your site, clicking Edit Profile then clicking Delete Account. The user can also navigate to YOURSITE.COM/?pn=edit_account (be sure to fill in your site's URL).
As a note, for users with any paid membership, deleting the account will remove the benefits of subscription.