Related Clues:

• Forbidden expressions of gratitude and disapproval
• Rising cricketer gets pair? Not to be mentioned
• Both appreciation and disapprobation are forbidden
• Prohibited practice
• Not allowed cheers? I don't like that!
• Modern composer's rules of grammar: double negatives are a ___ ?
• (Something) forbidden
• Impossibility
• (Irrational) prohibition
• Something totally unacceptable (informal)
• Wrong numbers
• "The Sweetest ___" (Sade hit)
• Possibility that's not an option
• Something that must not be done
• Out of practice Customs won't allow it