Websites that disable copying through the clipboard are annoying and their authors do not realize how impolite it is to interfere with functions that should be outside the control of a web page. It does keep a population of users from stealing information, but it also upsets legitimate users and is relatively easy to circumvent in many cases.
A copy-disabling technique I came across today involves the use of the oncopy event:
<div oncopy=”return(false);”>stuff you can’t copy</div>
Not only do these techniques offer content publishers no protection whatsoever (think: if you send it to the client, the client must have access to it to display it), they also keep visitors from coming back. It’s like disabling right-click… only worse. Don’t do it.