Believe me the girl will not want you to get rid of your erection and you should not try to mess with your hormones in that way. I'm not implying she wants to be with you but I think she would most likely be flattered if she even noticed it. If you do proceed with that plan, you may find you lose the ability to ever get erections. I also know of nothing that would do this for you.
If it imbarrises you, you might try masturbating right before meeting with her, that might slow down things. Erections are a good thing, but when they happpen at unwanted times, they can be imbarrising. If not masturbating, where loose fitting pants with a jock-strap, or tighter briefs! Something safe, I'm sure you can think of something..Good luck..