It's impossible.
You can't stop someone dragging their mouse to select a paragraph, then ctrl-c to
copy and ctrl-v to paste.
If you don't want other people getting their hands on it, don't put it on the
web.
You don't specify what you don't what people to copy e.g. is it the text in your
web page or images, pictures, music files etc.
Pretty much anything that seems to prevent people from right-clicking in their
browser can be worked around, besides, any web page viewed in a browser is
downloaded to the cache on the user's computer.
