I can't control what they're having at friends' houses, but we rarely have it at home. I don't drink anything other than water, tea, milk and the occasional Horlicks or fruit juice, so they're not normally bought anyway. I think this makes it much easier to limit.
My dc are allowed herbal tea, water, milk, black/fruit tea and the occasional coffee (now that they're in their late teens). Mostly they stick to water, milk and the odd tea or weak, milky coffee.
I've asked them to stick to 1 fizzy drink a week (max) and explained my reasons for this. I trust that, for the most part, they do this.
Occasionally we have fizzy drinks in the house (when bought for guests).
For example, we still have Coke Zero, Pepsi Max, etc, left over from a New Year's party and they will occasionally ask to have one.
Mostly they're there for their friends or other guests though, and once gone I won't be replacing them unless we have another get together.