HTTP Referer blocking with URL rewrite

Hi

I'm trying to block referrer spam coming through to my site and I've configured the URL rewrite section of my web.config as follows (inspired from Scott Hanselman's blog on blocking image hotlinking http://www.hanselman.com/blog/BlockingImageHotlinkingLeechingAndEvilSploggersWithIISUrlRewrite.aspx):

<rewrite><rules><!--Referral Spam --><rule name="Blacklist block" stopProcessing="true"><match url=".*" /><conditions><add input="{HTTP_REFERER}" pattern="^(?:https?:\/\/)?(?:www\.)?([^\/]+)(.+)?$" /><add input="{DomainsBlackList:{C:1}}" pattern="^block$" /></conditions><action type="AbortRequest" /></rule></rules><rewriteMaps><rewriteMap name="DomainsBlackList" defaultValue="allow"><add key="google-chrome-browser.com" value="block" /><add key="semalt.semalt.com" value="block" /><add key="semalt.com" value="block" /><add key="buttons-for-website.com" value="block" /><add key="free-share-buttons\.com" value="block" /><add key="4webmasters.org" value="block" /><add key="buttons-for-your-website.com" value="block" /><add key="buy-cheap-online.info" value="block" /><add key="best-seo-offer.com" value="block" /></rewriteMap></rewriteMaps></rewrite>

Despite this OK with Fiddler's HTTP request composer I'm still getting spam referrals coming through.

Could anybody shed any light on where I'm going wrong here?

Thanks ! :)

Chris