Possible Clues:

• 'Forget it!'
• "Forget it!"
• 'Uh-uh!'
• 'Ixnay' (or a conundrum in a tube?)
• 'Can't do it'
• ''Forget it!''
• 'Outta luck!'
• ''Nothing doing!''
• 'Fat chance!'
• "Out of the question"