You know, I didn't try, they are animated in the background and not static but I'd be quite surprised if they let you murder the balloon riders lol. Time to take a sniper out there and try
So, after bringing in a sniper and getting to a high balcony with a vantage point of the closest balloon:
I can hit the balloon with the sniper, and it actually has a hit effect and I can hear the bullet impact the material, but after blowing through both mags that come with the rifle, the balloon did not go down.
I can also shoot the passengers of the balloon, who also have the same hit effect, but they are not bothered at all by taking sniper rounds to the face.