If that could be achieved then, yes you would be quite correct. However, it is very hard to completely cut people off on the Internet. Furthermore, they can start disparaging you through other mediums (eg blogs) and influence people they still know to be using your site.
So? Thats free publicity. If they something bad about you, all they're doing is informing people of your existence. People will judge for themselves whether you are worth using, usually by checking it out themselves. At least, the kind of people you want on HN will.
The problem isn't with them badmouthing you on your site, it with them polluting your product. If the product sucks, people won't use it, no matter how much you suck up to them.
Once your site is big enough, it won't really be a problem.
In fact, it's unlikely to ever be a problem: if your site is small, nobody on a bigger site is going to care about a someone bitching about it, so the story will get buried. If it's large, any site worth its salt will delete a griefing post to avoid the inter-site grief (see: reddit deleting the posts that lead here, mefi deleting pretty much anything leading to LGF)
