RewriteCond %{REQUEST_METHOD} ^POST$
RewriteCond %{HTTP_REFERER} !^http://example.com%{REQUEST_URI}$
RewriteRule ^/comment/reply/([0-9]+)$ - [F,L]

Log entry of what I want to block:

1.2.3.4 - - [14/Feb/2007:05:28:11 -0800] "POST /comment/reply/127 HTTP/1.1" 200 36218 "http://example.com/comment/reply/215" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; MyIE2; Maxthon)"