Name: No UserAgent, No Post
Version: 1.1
Description: Denies POST requests by blank user-agents. May prevent a small number of visitors from POSTING.
Rules: 
RewriteCond %{REQUEST_METHOD} =POST
RewriteCond %{REQUEST_URI} !^%relative_root%(wp-login.php|wp-admin/|wp-content/plugins/|wp-includes/).* [NC]
RewriteCond %{HTTP_USER_AGENT} ^-?$
RewriteRule .* - [F,NS,L]