Agree with the posters above. One other point I'd add is if the negative comments do have some validity behind them, why not address the user and publically make an effort to resolve their complaint.
It depends upon for what reason they are there. If someone is spamming about you, then contact the administrators. If you've spammed and are now wanting to remove it, forget and move on. If there are complaints about your service, then face up to them. If it's the fact they are coming up in search results that are relevant to you, then you need to start a positive SEO campaign (if you're not sure what one is, search Google for seo reputation management and read some of the professional blogs about it).
I would recommend you to enable the comment moderating option to review and publish the comments which are submitting by the blog followers or anonymous. This will help us in preventing negative/spam comments easily.