.htaccess not blocking a specified referrer

I'm using .htaccess to block web visits from unwanted referrers.  However, I still get occasional visits, most recently from www.housesittersinternational.com   Can you explain how this is happening, and how to remedy it?

#Block traffic from "bad" referrers
RewriteEngine On
RewriteBase /

RewriteCond %{HTTP_REFERER} make-money-online\.7makemoneyonline\.com [NC,OR]
RewriteCond %{HTTP_REFERER} 245\.semalt\.com [NC,OR]
RewriteCond %{HTTP_REFERER} best-seo-solution\.com [NC,OR]
RewriteCond %{HTTP_REFERER} best-seo-offer\.com [NC,OR]
RewriteCond %{HTTP_REFERER} 100dollars-seo\.com [NC,OR] 
RewriteCond %{HTTP_REFERER} buttons-for-your-website\.com [NC,OR]
RewriteCond %{HTTP_REFERER} Get-Free-Traffic-Now\.com [NC,OR]
RewriteCond %{HTTP_REFERER} semalt\.com [NC,OR]
RewriteCond %{HTTP_REFERER} housesittersinternational\.com [NC,OR]
RewriteCond %{HTTP_REFERER} buttons-for-website\.com [NC,OR]
RewriteCond %{HTTP_REFERER} success-seo\.com [NC,OR]
RewriteCond %{HTTP_REFERER} semaltmedia\.com [NC]
RewriteRule .* - [F]

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