Pass around a sign-up sheet for the year. If there is a month when no one signs up, then there are no treats. No begging. No threats of not having treats if no one steps up to sign up. No whining. No judgments. Must keep the message positive or people sometimes are just not responding because they don't like the mean-sounding voices begging.
Then again, maybe the economy has just finally hit some folks and buying snacks is just an unwanted expense.