Related Clues:

• Banned thing
• ... by Italian one either way in London, on order
• Drink nothing! That would be futile!
• Numbers of a nonstarter
• Prohibited practice
• Modern composer's rules of grammar: double negatives are a ___ ?
• Impossibility
• Something totally unacceptable (informal)
• Cancelled rocket launch, e.g
• Wrong numbers
• Possibility that's not an option
• Something that must not be done
• Second helping, to a dieter
• 'What were you thinking?!'
• Taboo, to a child