incrediBILL - 7:01 pm on Mar 28, 2012 (gmt 0)
They are not hotlinking; full-size graphics are hosted on the Pinterest servers. Will that code work if they're not hotlinking?
They have to be able to download the image in the first place regardless.
You can stop images from being loaded with no referrer, block the Pinterest IP range, lots of options.
I agree with @greenleaves above, watermark all your images and then get free advertising.
Turn your infuriating problem into a marketing windfall.