SearchEngineWatch reported at the end of March that Yahoo had removed the ability to block specific sites from SERPs in their personalized service, My Web.
Previously, when you pulled up a search results page, each link would have a link available to "save", "more from this site", and "block". Now, the "block" links are gone.
They've now updated their FAQ for My Web, to state:
What happened to the block feature that enabled me to block any site from showing in my search results?
We have removed this feature from My Web. Previous users of the block feature can still access their blocked sites by clicking the Blocked Pages link under Go To in the My Web management Pages or Tags tabs.
I believe that they may have revoked this feature because black-hats found some security holes which may've allowed them to block some results from appearing for many users, allowing the sites of the black-hats to come up higher than normal in those users' SERPs.
This feature was really useful to users, and Google was anticipating doing something similar. I can't see any other logical reason for them to've revoked the functionality.