Stop Forum Spam has an API that allows you to query their spam list using a REST call. I've written a simple wrapper for their API and created a HttpModule that checks the clients IP against the spam list. Making http requests is very expensive so the result of the IP check is cached for 24 hours making the performance impact to the end user only slightly noticeable on the first request.
The download includes the http module dll, test website and a couple of unit tests.
When I get a bit more time I'll look into improving the performance, perhaps async using requests. Now off to finish my online xmas shopping.
Download Code
Download DLL