Proxy "Open Relay" used to send messages - Apache2

Hello All,

I'm having problems with "Open HTTP Proxy" Relay. Some spammers are
using my proxy to send messages with other mail servers (my proxy is
"trusted" for this mail servers). After tests, I concludes that the
method used for this is a connection by "http-post" (tested with
pxytest.pl (http://www.unicom.com/sw/pxytest)). If I block "http-post",
probably my proxy will not function. Someone has this problem? How to
solve this.
My proxy configuration (it's an Apache2):
"
<VirtualHost 10.1.144.3:80>
        ServerName msexchange.xxxxxxx.com.br
        UseCanonicalName On
        ProxyVia On
        ProxyRequests On
        ProxyPass / http://10.2.144.4/
        ProxyPassReverse / http://10.2.144.4/
        ProxyPreserveHost On
        ProxyPass /exchange/ http://10.2.144.4/exchange/
        ProxyPassReverse /exchange/ http://10.2.144.4/exchange/
        ProxyPass /exchweb/ http://10.2.144.4/exchweb/
        ProxyPassReverse /exchweb/ http://10.2.144.4/exchweb/
</VirtualHost>
"

Regards,

Fabiano Felix