Bitt, You da man.

Rebooting the proxy server worked.

I discovered quickly that I needed to use the "0.0.0.0" form of the new hosts file instead of the 127.0.0.1 form. If I used the latter, then the end-user is prompted to authenticate with the proxy server.

But after changing it to 0.0.0.0 and rebooting the server, all is well. Each ad box comes up as a broken link now.

And, this happens to benefit the users on the rest of my network, too. Rock.