Right now you can't delete users from the myTurn platform to ensure your site remains in a consistent state and you don't lose history. You can "deactivate" them which hides them by default:
- Search for and click on their username
- In the Upgrade or Renew Membership section of their User Details change their membership type to "Deactivated"
- Click the "Change Membership" button in the membership section of the page
The user should no longer come up in searches unless you specifically do an Advanced Search and check off "Include Deactivated".