Hi ovalking, It is stunningly easy!
Create an file called "hosts" with a texteditor. For "customizing" the DNS put one domain per line into this textdocument followed by ". A 127.0.0.1" (including the period!). Save this document inside the preferences folder or the system folder (both should work). As soon as TCP is initialized the file is parsed, and you can "redirect" all the crap into Nirvana! Technically it is a loopback at "localhost", means the OS detects that this domain request shall never be sent to the internet, but to the own computer. You can also use the IP adress "0.0.0.0" so that absolutely nothing happens.
Here is a recent (still maintained!) list with advertizing domains, ready in Mac OS 9 format (unix and win hosts-format is a little bit different). Just copy this list, ad all the facebook-, twitter-, google-crap to the list, and save the file inside your preferences.
http://pgl.yoyo.org/as/serverlist.php?hostformat=machostsMore informations:
"Open Transport/TCP supports a Hosts file that may be used to supplement and/or customize the Domain Name Resolver's initial cache of information. The Hosts file is found in the System's Preferences folder. This file is parsed when Open Transport/TCP is initialized. As in MacTCP, the supported Hosts file features follow a subset of the Domain Name System Master File Format (see RFC 1035 page 33)."
https://web.archive.org/web/20000831101418/http://til.info.apple.com/techinfo.nsf/artnum/n75085"Under Mac OS 9 and earlier there is a hosts file in the root level of the system folder. If you're using Open Transport it can also be located inside System Folder:Preferences, although either will work. Or you can just open the TCP/IP control panel and go into Advanced under User Mode in the Edit menu. Then you have the option of choosing whatever hosts file you like and it will be put where it needs to be. I've also been looking at the syntax for host files under this system, and you should use for this either SITE_NAME. A 127.0.0.1 (the period is required) or SITE_NAME CNAME 127.0.0.1."
http://pgl.yoyo.org/as/news.php#macinfo