try by installing mod_security...
If you have it installed just apply this rule at your htaccess
Code:
<IfModule mod_security.c>
# Sends matching requests a 405 Method Not Allowed Status Code
SecFilterSelective REQUEST_METHOD "!^(GET|HEAD|OPTIONS)$" "deny,auditlog,status:405"
</IfModule>
This should block ALL POSTS and will show 405 error.
I have used mod_security but never used this rules so I don't know if it works.
I found those rules on this page (modified it a bit to suite your needs)
http://www.askapache.com/htaccess/mo...ss-tricks.html