Releasing these balloons is littering. It's exactly the same as throwing the empty balloon into someones garden or into a river. Get people to perceive it as littering and most will stop releasing balloons, or will no longer perceive it as cool or pretty. Make it seem gross. Shame any events that do this on instagram - that's probably a good start.
Then maybe we can get started on cigarette butts, the one thing that is bizarrely socially acceptable to litter anywhere - even in places with a strong social stigma for other littering.