Some blogs are receiving a lot of spam comments. One of the simplest ways to avoid wasting all time to cancel and mark as spam is to ban the IP. A nice plugin wrote by Lester Chan is WP-Ban
» Ban users by IP, IP Range, host name and referer url from visiting your WordPress’s blog. It will display a custom ban message when the banned IP, IP range, host name or referer url trys to visit you blog. You can also exclude certain IPs from being banned. There will be statistics recordered on how many times they attemp to visit your blog. It allows wildcard matching too.