In Internet Explorer, I don't know if there is a Registry entry to set the
referrer permanently (Google searches seem to indicate there is not), but
lots and lots of "firewall/popup blocking/browser security" type software
(Norton Internet Security, Window Washer (?), Proxomitron, etc) that can do
it.

In Netscape 4, you edit prefs.js and set
user_pref("network.sendRefererHeader", false);

In Opera 7.54, it's Tools > Preferences > Privacy, uncheck Enable referrer
logging.

Basically, it's easy enough in all browsers (and in fact some users
probably block it without even realizing it) that it can not be relied on.