Comment Spam

COMMENT SPAM….I’m curious about something. Dan Drezner writes that he’s upgraded to a new version of Movable Type that required him to get rid of MT-Blacklist, and as a result he’s now deluged with comment spam. But it strikes me that an easy way to eliminate spam would be a plugin that prevents posting of any comment that contains a URL. If you want to be a little less draconian, perhaps the plugin would allow a maximum of one URL per post. Since comment spam all contains multiple URLs, this would get rid of the spam.

Has anyone written a plugin like this? Or am I missing something and this wouldn’t work?