Today, while looking at the last tirade by the anonymous German commentator on the Whisher blog, I went out searching for a plugin that would prevent someone from even posting comments based on their IP address (yes, WordPress has this in the preferences, but it will only mark posts as spam, it will not block access completely).
Stumbled upon Bannage, a very nifty but simple plugin that compares the IP address of the visitor to a set of ranges (you can use wildcards too to ban whole classes!) and changes the HTTP response to a 403 Forbidden, thus preventing him from even viewing the blog. If you get a lot of spam from you-know-which countries, this plugin is very well worth considering.