AR 600-8-2, paragraph 1-32, says that the SF tab may be revoked, for among other reasons, if a Soldier...
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I did not do a close reading of the reg and there may be a provision stipulating what can and cannot be done to people who have retired or separated. However, a plain reading of the reg makes it clear that it could happen, regardless of whether or not someone is currently service. I don't know of any cases where this has been done, but this is also not something I follow closely.
Regarding Murphy, I think a case can clearly be made that he falls into the above category, especially the way he trashed SGM RItter and flat out refused to rectify it. But as much as I would like to see him stripped of his tab, I worry about the precedence it would set and how it would be politicized if the Army started doing it to people who are no longer in the service.