Recently I have had tons of referral spam in my blog logs. I didn't want to mess with IIS filters or modifying the dasBlog source code. I planned on writing a HTTP Module to check the request against a set of rules to help block invalid request when I found ReverseDOS. It lets you specify a set of filters as regular expressions and if the request matches any of the filters it blocks the request. SubText (another Open Source ASP.Net blog engine) ships with ReverseDOS and I'm not sure why dasBlog doesn't.
So far this is working quite well. I had to tweak the filters a bit to match the spam I've been getting and haven't had any issues so far. Once my filters are tweaked out I'll post my configuration file.