If you don't feed the negativity, it usually goes elsewhere to pick a fight with someone who WILL fight with it, and thus feed it.
Just to be clear, all posts by said downer will be removed. I am not even going to give thought to whether or not it is a valid point-I simply do not want this person hanging around. For my part, I will not comment on any comments on any other person's blog where I may encounter this being. In effect, it will be a self imposed "block" button until blogger comes up with one of its own.