Possible Clues:

• "Won't do it"
• Naysayer's word
• "You're dreaming"
• "Nah!"
• Slangy turndown
• 'ÔFraid not'
• ''Nothing doing''
• "Dream on!"
• Lack of exercise? Not on your life!
• Slangy denial, and a hint to 20-, 29-, 46- and 56-Across