I've got nothing helpful for advice. My own all weaned themselves between 18 and 20 months I think. It was such a non issue that I don't even really remember when they stopped. I think if you don't feel comfortable going cold turkey, you just have to be consistent. Maybe only allow it once a day, at a time when you decide?
I know this isn't the same thing, but my youngest is almost two, and once in a while her sister will be eating something that I can't let her have. I am a bit amazed, but when I explain "that treat has gluten, you will get sick if you have gluten", she almost always seems to understand. She just accepts it and moves on. I think no matter what you decide to do, just be consistent.