Use comments only when they're necessaryThink twice before deciding to enable a guestbook or comments. If this feature is not really necessary or you will not be able to monitor the guestbook and comments regularly, consider disabling it. If you already have a guestbook, check to see if it's useful to visitors, and if they've been visiting it. If not, consider ways to improve the comments/guestbook feature, or remove it. A lot of spam comments don't create a good impression. Most blogging software will let you turn comments off for individual posts.
Use anti-comment spam tool
Block comment pages using robots.txt or META tagsYou can use your robots.txt file to block Google's access to certain pages. This won't stop spammers from leaving comments, but it will mean that links in these comments won't negatively impact your site. For example, if comments are stored in the subdirectory guestbook, you could add the following to your robots.txt:
Disallow hyperlinks in commentIf you have access to the server, you may want to change its configuration to remove HTML tags inside your guestbook. Spammers will still be able to leave comments, but they won't be able to publish active hyperlinks.
If comment spam is driving you crazy, try one or two of these methods and see how it works out. Got comments or suggestions? Leave them in our Google Webmaster Help discussion group.
we always try to ban spam from the web, at the same time, we also need to consider why spam exists