Lately I have seen more than a few "why is this here?" type comments on topics that have already gone popular on HN. The bottom line is that happens because not everyone has the same interests, and this is a social site with many different people on it.
If the fact that you have to share this wonderful digital playground with the other kids really, really bothers you, then this is for you. It's a Greasemonkey script called "HN Toolkit":
http://userscripts.org/scripts/show/25039
Part of the functionality is the ability to blacklist submissions, either by keyword in the title or in the domain (or entire domains, whichever way you want to look at it). It cycles through the list of stories and matches each against the blacklist, adding style = 'display: none;' to each match found.
Hope this helps.
Every online community, from the very beginning, has faced its own crisis of identity as it scales. Perpetual September is as inevitable as the second law of thermodynamics.
I long considered HN to be a counterexample precisely because the group is not only self-selecting, but constitutionally disinclined to enjoy shallowness. A group of true geeks and entrepreneurs does not have to be coerced into expressing a love of Erlang internals and a hatred of celebrity gossip.
But, inevitably, any given community will attract people with different interests. A war of sorts inevitably occurs as both subcultures clash and try to gain power. The result is one we've seen in every single online community that has ever been created.
imho, the culture of HN is smart enough to either find a solution or at least a way to delay this process. There's a reason PG tells us not to complain about "redditization": such complaints are the very thing that dissolves a sense of community and shared values.