How do I block Semalt on IIS

Checking my Google Analytics I'm getting a lot of traffic from semalt.com. I know this is a traffic spam tactic, so I want to completely block their requests and stop it from turning up in my traffic stats.

I read here http://logorrhoea.net/2014/01/how-to-block-semalt-com-referrer-traffic-using-htaccess/ that on Apache it can be done like:

# block visitors referred from semalt.com
RewriteEngine on
RewriteCond %{HTTP_REFERER} semalt\.com [NC]
RewriteRule .* - [F]

but I need it for IIS, probably via the web.config, but how to do it?