I found from another post that stated Access Protection must be disabled in the McAfee Security Center before the Quarantined items can be deleted and then reenabled after deletion. This fact was left out when the original post for deleting the items in C:ProgramData\Mcafee\VirusScan\Quarantine (a hidden folder) was made.
I put some instructions for this in https://community.mcafee.com/docs/DOC-2973 ("Manual deletion of quarantined items"). Did I leave something out?
I take your point about the folder being hidden. I've changed the wording of that section to make it clearer.