You can stop them if they are under 18 and you are their legal guardian. I have this debate with my wife all the time. My son is almost 9 and she still doesn't feel that he should be playing football. He plays soccer all year without any pads or helmets and somehow that's okay in her mind. Some battles you just can't win. At least she's agreed to let our 6 year old daughter play lacrosse. Baby steps.
Honestly I like both Toomer and Warner and see where they are both coming from. This is one of the rare issues where I really can't chose a side.
Man I don't like soccer lol, rugby is a much better sport. I know that you have a legal obligation to give permission for your child to do things by a consent form, but what I'm saying is, if I had a kid I would probably sign that letter to let him play if he really wanted to. Can't say the same about the mrs's!