I think the only way is blocking proxys with a proxy shield
Please add this to ur .htacsess
RewriteEngine on
RewriteCond %{HTTP_REFERER} ^http(s)?://(www\.)?a-bad_proxy.com.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http(s)?://(www\.)?a-bad_proxy.com.*$ [NC]
RewriteRule .*
Google [L]
and test and reply back if it don't work