two things to remember:
1)they will block any link to stock photo sites of your portfolio
2) they use your photos without permission on their own sites
eg. did anyone see the weather sites? there's flickr photos
being used. i wonder if the photographers know that.
i was shocked to find that out.
I tried and Dreamstime is blocked. It looks like Shutterstock and ISP are fine, but I'm not 100% sure. Alamy and Photoshelter definitely work.
For me, being completely new, putting some of my images on Flickr is some form of pubblicity and I can take the risk of seen my (poor) images being stolen. I'm not that good to be worried about that now

More seriously, do you need a Pro account to disable downloading?