helleborine - 1:08 pm on Oct 29, 2012 (gmt 0)

Visitors from Pinterest show Pinterest as the referrer.

Pinning actions are identified with Pinterest in the user_agent field.

You can use htacess to control what image pinners are pinning by blocking Pinterest as user_agent.

RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} Pinterest [NC]
RewriteRule .*\.(jpe?g|gif|bmp|png)$ http://imagehostingwebsite.com/copyrightwarning.jpg [R]

You can't host the copyright warning image on the website you're trying to block from pinning.

The pinner will try to pin your image, but they'll pin the redirected copyright warning image instead.

One downside is that it is my belief that Pinterest linkage is super-good for Pinterest, but hurts your rankings. The substituted image creates nofollow links to your website, and many more Pinterest inbound links designed to turn Pinterest into a SERP powerhouse without sharing any of the 'juice' with you.