What you want is a "Banner" (see
here and
here for some details). Basically, when anyone tries to log onto your system they are presented with a banner that details the terms of service...to get past that banner and log on, they have to press a button that you can lable "I accept these terms". That covers you from a legal perspective.
You may want to use
PAM (on Unix/Linux systems) to control who can log on and what they can do.