Similar to the problem of WikiSpam, blog spam is a form of link spam which is placed in comments on people's blogs. Wired has a short article about the problem. (Note this is slightly different from the problem of Splogs, which involve spammers creating whole new blogs full of spam)
Generally blog spam is more widely known than WikiSpam, because blogs are more widespread than wikis. As such there are is a larger community of people tackling the problem of spam in blog comments.
Chongqed.org has tended to focus more on WikiSpam, but many solutions for blogs would also work on wikis. You will find discussion of spam solutions on the WikiSpam page and also AntiSpamRecommendations. Also the idea of stealing a spammers pagerank with Chongqing Links still very much applies to blog spam, so chongq those spammers!
There are other anti-spam solutions which are apply only to blogs and are just not usable on wikis. We can discuss these here.
Blog spam has declined a lot in the short time since the start of chongqed.org. Most blog software and services now have pretty good spam prevention options. If they aren't enabled be sure to take a look at your options. And if you are using an old version with little or no antispam features, upgrade your blog software to the new release which likely has better.
Only allow comments on recent posts. Often spammers find old pages through Google and spam a lot at once. This shouldn't affect regular users too much since most will be reading current posts.
Block URLs totally or make them text only (not links). This won't work on a wiki but on a blog I think it is a reasonable solution.
Blogger's solution is to redirect all URLs in comments to make them useless in increasing the URL's PageRank. This also strips off the referring site, which some people don't like, but its not a big loss.
Google introduced a solution that does not require redirects. By adding the rel="nofollow" attribute to links in blog comments, Google and other search engines will not use the link when ranking pages.
A simple way to block ReferrerSpam? is don't display referrers. They aren't vital parts of the blog. If you must have them only display them for a short time and only the most recent ones. This way even a site with low traffic will still clear out spam referrers hopefully before Google notices the links.
Setup or use an existing redirector system for links like blogger does.
scriptygoddess.com has a lot of good antispam plugins among other for WordPress?. It also has a small blacklist.
Need to mention Pivot blacklist and BadBehavior?.
I made a blog page on my website which includes a comments form for people to complete if they wish to add their comments. The entries are stored in a flatfile and they are only added to the website if they are approved by me. I had to take this approach because my site is visited by children. I really don't want sick people leaving links to sites on incest etc. I get loads of attempts by spammers to put their rubbish on my site but I only have to transfer any sensible comments to the site and then overwrite the file to get rid of the spam. On the guestbook I removed the facility of adding any links to the entries so that I don't get any spam. The worst I get is an entry that says "error." It's not very pretty in the guestbook and it gets deleted but at least the kids are safe. I'd like to have a wiki on my site but I'm undecided as to whether it's worth it because of the spam. I may just include a forum instead. The problem is that people are asking lots of questions through the guestbook when they would be better off using a wiki or a forum. Best wishes Pat