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.
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)