There are at least two methods used by tree companies to prevent a tree from flowering. One is by using a spray-on chemical with the trade name of Florel Growth Regulator (ethephon). This chemical causes fruit to drop before it sets and must be applied at full bloom and at the right temperature. The problem here is that even professionals have a hard time determining optimum spraying conditions.
The other method I found is using a deflowering hormonal solution applied internally as a basal (tree trunk) treatment. The chemical's trade name is called Snipper (Indole-3-butyric Acid). According to the label, this acid solution is for use by commercial applicators only on "sweet gum trees in commercial and residential landscapes" where sweet gum trees are grown. This microinjectable chemical also has to be applied only at a certain critical period of time.
You just might want to join the About Forestry Forum's discussion on applying chemicals to stop tree flowering.
Join the Discussion
Sweetgum Fruit Photo - by Steve Nix, Licensed to About.com