That would require some NAML coding skills to edit the appropriate macro.
However, the alternative that I use on my forums is, at the top level, turn off all User Groups at:
Options > Users > Change Permissions > Show_group_members
and then have a hidden sub-forum that only an Administrator can access where I turn on all User Groups.
To make a sub-forum that is completely hidden to certain user groups use:
Options > Application > Extra & add-ons > Content > Smart application pages
If you don't use that feature a sub-forum will be visible but those without the "View" permission will encounter an "Access Request" screen.
One of the reasons I keep a hidden sub-forum for administrators is as a place to move unwanted posts, replies and comments. That way I can keep any evidence of what was written and may be required later. It's also a good place to store early drafts of posts. Than I can copy and paste them them into the main forum when they are ready to go.
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