This. Too often people think they can't just say "no" and they have to offer an excuse to go with the "no". But this isn't so. I kind of blame this all on that political correctness 'stuff', whereby no one is ever allowed to be offended - by anything, anytime, anywhere, anyhow.

"Can you grab my bag?" "Sorry, no."

"Can you change seats?" "Sorry, no."

"Can I swap my ham sandwich for your gold-leaf topped caviar blinis?" "Sorry, no."

It's not rude to say "no", so drop the fear-of-offending 'stuff' that has you scrambling for an excuse. But you could always give a smile.