A customer sent me and a few coworkers bottles of wine as a holiday gift. When the one coworker came to give me the bottle I simply said, "I don't drink anymore, would you like an extra bottle?"
His response was, "Oh, wow, thanks!"
That's all, no "why?"
Of course, I still wrote a note to the customer thanking them for thinking of me during the holidays.
If you are polite and just say "no thank you" people usually respect your boundaries without questions.