Blocking thousands of IPs with htaccess will have a negative impact on your site's loading speed. Better consider blocking some HTTP protocols proxies use with htaccess like the guy describes here: http://perishablepress.com/block-tough-proxies/.
<ifModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP:VIA} !^$ [OR]
RewriteCond %{HTTP:FORWARDED} !^$ [OR]
RewriteCond %{HTTP:USERAGENT_VIA} !^$ [OR]
RewriteCond %{HTTP:X_FORWARDED_FOR} !^$ [OR]
RewriteCond %{HTTP:PROXY_CONNECTION} !^$ [OR]
RewriteCond %{HTTP:XPROXY_CONNECTION} !^$ [OR]
RewriteCond %{HTTP:HTTP_PC_REMOTE_ADDR} !^$ [OR]
RewriteCond %{HTTP:HTTP_CLIENT_IP} !^$
RewriteRule .* - [F]
</ifModule>