reporting most likely isn't gonna do anything. if you are getting that volume of mail, then most likely it's a bot or some program.
the best thing to do in this case is probably setup an image verification number. by that i mean have php generate an image that has a number or letters on it, and have the user type in what the image has.
then, before you hit the mail function, make sure the number the user inputted matches what was on the image.
here's a tutorial i found real quick on google:
http://www.phpnoise.com/tutorials/1/2