I have enough of people trying to login to my vBulletin admincp but fail.Each failed login is logged and emailed to me.It's nearly always coming from ip address that are "tor exit routers"

also i see in my access logs those ip address make a POST request to login.php?do=login&do=login what is the point of that ?? the login will always fails if the referer is not my domain... so in short how do i get rid of tor anonymizing proxies? there is a htaccess blocklist but that's way to large and will cause heavy serverload any thoughts ?