Use the following .htaccess snippet to block the Semalt.com crawler from crawling your site.
## BLOCK SEMALT ##
RewriteEngine on
RewriteCond %{HTTP_REFERER} semalt\.com [NC]
RewriteCond %{HTTP_REFERER} ^(.*)\.semalt\.com [NC]
RewriteRule .* - [F]
## END BLOCK SEMALT ##
