In your example, someone advocating racism can be countered by stating clearly and simply the historical, anthropological and economical reasons why racism is wrong.
Politeness implies reasoning. An internet discussion is not war, it is a conversation. Whenever you leave politeness aside, you have lost. Always. (Obviously, this is my way of seeing it).
In your example, any pro-racist argument is by itself both impolite (because it insults persons straightaway) and wrong (for many reasons). It is a no-no in any case. If you fall on the same side of the wall (by being impolite) you are placing yourself at the same level and your "reason" is lost to the reader.
You can say: "racism and eugenics have lead mankind to the Nazi extermination camps" and being polite.
To repeat myself: politeness is not lack of clarity. It is just the way to properly oppose wrong ideas without harming people defending them. At the same time, it protects your argument.