Can web publishers prevent copying of content?

Yes. With Images they can use Watermarks, with the site it self they can hide the source as shown here:

http://www.dynamicdrive.com/dynamicindex9/encrypter.htm

Also they can disable right clicking like on here:

http://www.dynamicdrive.com/dynamicindex9/noright2.htm

ArtistScope provide proper copy protection for all media displayed on a web page, safe from all copy including printscreen and screen capture.