Realistically speaking, i do not personally think it's a website's place to tell the browser what functions it can and cannot perform. Secondly, technically speaking, whenever someone views your site, they are actually downloading the content of that given page in order to view it, so they'll already actually have a copy before any right-clicking or selection is performed on the page.
If you're still certain you want such a script, this would be a good place to start in finding one.
If you need help implementing the script, i'd suggest a read of the following page.
NOTE: There's some useful links at the bottom of the codex page(linked above).