The short answer is you cannot delete a user account. Data attached to each account is either valuable to the athlete (results, diet etc), the gym (check-ins, payments. etc.), or both.
If the account is a duplicate, coaches have the ability to "Merge Accounts" via the Coach's Dashboard (both accounts must be set to the same gym).
If the account is for a user no longer at your gym, you don't need to do anything. They will not show up in any reports aside from "Everyone" and "Non-Members". If you REALLY don't want to see them in your gym at all, you can move them to Main Site via the Account Edit screen (update their gym). Once an account is not under your gym, you lose access to that account (aside from the public info).
If you are a user that wants to delete your own account, email email@example.com to request the account be disabled.