Lot of spammers may use the comment section of a blog to post spam messages and links that promote their websites.
Adding rel='nofollow' attribute to link will intimate the search engine that the particular link is
not audited by your site, and should not be counted as a trusted vote for the popularity of the link.

Although this definitely do not prevent spam, but it remove the motivation that results in
a spammer target your site. The provided link will work, but will not be as desirable to a spammer
as it offers a diminished link equity value.

HTTP status codes 301 and 302 codes are basically used for redirection which means another request need to be made in order to complete the HTTP request as the content is located elsewhere.So the web page which replies with either 301 or 302 HTTP status codes, the really do not return any HTML, but includes Location: HTTP header indicatingnext URL where content is present.

Cloaking is basically providing different content to a search engine then to actual visitors and is implemented through IP delivery. For cloaking, you need to store  the IP addresses of the various robots that hit your site. Then check if the current IP is a robot, and accordingly changes the presentation or logic.

There may be case when due to some terrible happening like database not performing, your web server return blank error pages 404 error. Perhaps the web server goes down altogether. In in the meanwhile, search engines are trying to index your pages, and donot find anything getting only blank pages.