you wouldn't be prosecuted for what you are saying, merely the method of delivery.
I don;t know what charges specifically are available to charge you with but I can;t see anything too great. Something like littering or something similarly minimal. It does cost money for the government to dispatch people to remove the banner that you may end up paying for as well.
Now, if there was an accident caused by the distraction you may cause, the possible charges could grow quite a bit.

