Thats a tough call for any parking program, and I dont blame the programs I blame the scammers who abuse proxies.
Most Schools/Government Offices/Corporations, block access to non business related sites.. sites like myspace, ebay, games sites and of course adult sites.
The simple fact is that most will use some sort of proxy to by pass the filters, this is also true from people in Coutries like China.
I would assume that 30% of the proxy traffic is genuine, but it would be hard to figure out which were the 30%.
There is a solution however, you can filter the traffic yourself before sending to a parking program, its easy enough to detect proxied users. You could then just transfer them to somewhere else.