В логах nginx вижу
173.245.54.234 - - [12/Oct/2015:10:38:40 +0300] "GET / HTTP/1.1" 403 142 "-" "-"
162.158.95.173 - - [12/Oct/2015:10:38:40 +0300] "GET / HTTP/1.1" 403 142 "-" "-"
173.245.54.234 - - [12/Oct/2015:10:38:40 +0300] "GET / HTTP/1.1" 403 142 "-" "-"
173.245.53.138 - - [12/Oct/2015:10:38:40 +0300] "GET / HTTP/1.1" 403 142 "-" "-"
if ($http_user_agent = "") { return 403; }
if ($http_user_agent = "-") { return 403; }