The antonyms for the term "stamp of approval" can imply disapproval or rejection. These could include phrases such as "not up to standard," "fails to meet requirements," "insufficient," "lacks credibility," and "unsatisfactory." These negative antonyms indicate that the item, entity or action that is being evaluated does not meet the explicit or implicit criteria for acceptance. There are also neutral or non-committal antonyms such as "unremarkable," "inconclusive," "untested," or "undecided," which leave the judgment on the item, entity or action ambiguous. In short, the antonyms for "stamp of approval" suggest refusal, disqualification, dismissal, or hesitation.