first i'll explain what I mean when I say something as generic as web proxies, I'm pretty sure 95%+ of you know what a proxy is so i'll skip that much, I know theres a little bit of data transferred (referral id, etc..) for management.. I have not looked too far into it currently. But this used to be the case

I am curious if it is possible to try and prevent proxies, such as a php script checking RBL lists to see if it exists and then blocking it, It would slow down things sure but if it was just on a reg script or something a few seconds while it does the lookup isn't such a big deal, I suppose another route would be adding a mod to apache that could do something like rbl lookups and then taking an htaccess type route, but thats not really appropriate for this subsection.

I want to know if this is possible with current PHP/ASP, etc.
or if anyone has any examples of such a thing being done, obviously searching for "proxy"/"proxies" on any common search engine reveals other results which are not so much useful.

Maybe if anyone can add any insight to this it would be much appreciated, Thanks!