Create table with owner set to the administrator of the database (NOT PostgreSQL SERVER!!!) and grant only the needed rights (or none of them) to the user from which the usual processing of the database will be performed. Then, create a trigger function with SECURITY DEFINER set and own it by the owner of the database (or other user, who's granted to UPDATE the table). So, if no one else is granted UPDATE on the table, the only UPDATE-modifiers of the table will be the owner and the trigger function's owner (if differs from owner).
Best regards, Andrey V. Semyonov
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