Kill Internet Ads with HOSTS and PAC Files: Using publicly available lists of ad servers, you can block ads by redirecting those domain names to an invalid IP via your HOSTS file.
Compiling a list of ad servers for an ad-blocking HOSTS file would take a lot of time, but happily you don’t have to do it. There are numerous ad-blocking HOSTS files available for download on the Internet. Mike Scallas distributes one that is updated each month.
I tried it, and it works…okay. It blocks some ads, though certainly not all. And sites that use IFRAMES to display ads have the ad replaced with 404 text, which is even more annoying.