I'm just going to re-iterate what acrylian said.. you can disable the context menu (in IE, you can even replace it; not too sure about the other browsers), but there's not much of a point in doing so.
( well, arguably, replacing it can extend functionality specific to the site in question, but anyway )
But if you must:
http://www.google.com/search?q=disable+right-click
Take any of those sources (usually javascripts), include it in your template(s), done.