How do we wean people off sweet things?
What about a little trial? I wonder what would happen if you stopped everybody coming out of the newsagents with a can of cola and, before they drink it, give them the sweetest peach you can find. And a little towel afterwards to wipe their hands on.
Firstly, check if there is a grin on their face.
Then ask them which they would prefer.

All you need is a few % preferring the peach, as long as they knew where to get hold of one like that, easily. I think you will find they are called greengrocers. You have somrthing to challenge every faux taste-testing cola ad.