I don't know what's happening on my website but I've noticed that hundreds of webclients are using my Apache server as a proxy or something.

In my access logs I see something like this
PHP Code:
A_REMOTE_IP - - [15/Jun/2013:16:04:55 +0200"GET http://externaldomain.com/ttj?id=1292198 HTTP/1.0" 302 0 
At the moment I have blacklisted 20 x /16 networks on my firewall so it solves the problem at the network layer but I feel there's something wrong with my Apache configuration because clients shouldn't be doing that.

First, I don't know how to describe the problem so I can't find a solution on the web. Would someone has an idea of what's going on ?

And second, how should I solve this ?

All these remote URLs that I'm seeing in my logs are mostly advertisements: banners etc.

Thank you