As you probably know, internet survers and browsers talk to each other using a protocol called http. The http requests coming from browsers would usually have a "referer" field that would indicate the page the user is comes from. Hence, if you have root access to the server, you could script it not to serve the request if the "referer" field is empty, or if it's the wrong url. That would not fool professional intruders, who could spoof the "referer" field, but would
keep off your usual web user.
The details of your implementation would depend on your webserver (Apache, IIE, etc), your language for server side scripts, and whatnot. And even i knew those I couldn't tell you the details, because it's been a while since I've been mucking around with cgi scripting. But this is the general approach I would take.