Part of the social-network-portability effort.
Many social-network-portability sites provide the user the capability to "block" or "ignore" other users of those sites, typically because those other users are stalking, trolling, harrassing, or behaving negatively in some other way.
The "block list" effort documents how current social networking sites are handling these interactions in the hopes that this research can be used for developing a portable block list that a user could privately share with other sites (probably via oauth or OpenID or both) so that they could block a troll on one service, and have the troll automatically blocked on other services as well.
The following services have a block or ignore feature.
- Flickr ("block this user" link when viewing a user's profile)
- Twitter ("block ((username))" link when viewing a user's profile, just under their friends list)
Previous attempts at block list formats:
- usenet/netnews kill files