The accounts you're seeing are the ones which you have parental control over, there isn't really any way to delete or remove an account from under the parental controls once set (until the account reaches 18 and can upgrade to Adult status) so
unfortunately there isn't a lot you can do.
You have the option to remove it from your hard drive if the profile appears there but it will still be linked to your account via the Parental controls. Perhaps one of your sons created another account?
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