Were airworthiness certificates revoked for B737 Max? Following up from this question - or more specifically Brian Drummond's comment on the accepted answer:


  Or even revoked without modification, if something new is discovered about the existing components (e.g. failure modes)


When B737 Max were grounded due to suspected issues with MCAS, what happened to the Certificates of Airworthiness of individual aircraft?  Technically speaking,


  something new [has been] discovered about the existing component


Per the quoted comment, this could result in the certificates being revoked.  Did it actually happen? If not, what formal measures exist that prevent somebody flying a B737 Max? (Other than common sense of course.)
The grounding of the 737max is a Prohibition of Operation, not a revocation of Type Certificate. This is done by each jurisdiction around the world. China suspended flights two days before the FAA did.

Technically, if you could find a country that never prohibited its flight, the Max could still be used there. Repositioning flights of grounded aircraft, as to a storage location, are authorized on a case by case basis.