It's going to be difficult to prevent kids from coming across that stuff on the internet, since it's pretty much full of it.
A lot of it is about breaking taboos and pissing people off anyway. If, as a parent, you react overly hysterically to it, then you're probably only going to make it worse. It just happens to be in the nature of teenagers to rebel against the parents, and today's "counter-culture" is what it is, a reflection of mainstream culture, in many ways. If "Nazism" and "white supremacy" are the biggest taboos, then obviously the biggest taboo-breakers will adopt those ideals as their own, if for no other reason than to cause maximum outrage.
The best solution, in my mind, is to be more sensible and less sensitive.