The best thing you can do for comments like that (esp. when they're made by "green" accounts) is to just downvote and ignore. They'll be downvoted to grey, then deleted. The accounts will be hellbanned in time (comments autmotically deleted, posts only visible to poster). Replying only clutters the discussion.
For some users, you can click on the permalink to the comment to see a "flag" option.
"Please don't submit comments complaining that a submission is inappropriate for the site. If you think something is spam or offtopic, flag it by going to its page and clicking on the "flag" link. (Not all users will see this; there is a karma threshold.) If you flag something, please don't also comment that you did." - http://ycombinator.com/newsguidelines.html
Hellbanning is precisely why I have showdead on. There have been some notable users (creators of products and various websites) whom they were hellbanned for no apparent reason. Having showdead on shows even them, however you still cannot comment to them directly.
I think I've only seen 2 or 3 spam posts on dead. And a few 'special people' whom are hellbanned. In a nutshell, the S/N does not get noticeably worse with showdead on.
