Apple - Delete a standard user from Mac OS

The – button is greyed out for a user when the user is still logged in with Fast User Switching. Log out the other user you're trying to delete, then return to System Preferences with this user account and try again. You can restart to be sure that user is inactive as well.

You can delete a user from command line and you need to be careful you have the exact short name correct or you will erase the wrong data. In this case, the short name is mac. Run the following to delete the user and separately delete the user's home folder. If you want to keep the home folder, don't run the rm command.

sudo dscl . delete /Users/mac
sudo rm -rf /Users/mac

If the dscl command fails, reset the password on the account and retry the above commands.

sudo passwd mac

I had a very similar problem to this. With the help of Apple Support I solved it. I posted the solution here: https://discussions.apple.com/message/33849669

Repeating that here, it is:

• Boot into the recovery partition.
• Choose Terminal from the Utilities menu --> Terminal should appear.
• Enter: resetpassword at the prompt and hit return --> a new window/app should appear behind the Terminal window called "Reset Password"
• Go through the steps to reset the password for the account you're trying to delete.
• Restart the Mac and log back in as your main admin account.
• Go to Users & Groups System Preferences pane, select the account you're trying to delete and hit the minus button.

Fixed it for me. Hope this helps!