There are three console commands that govern who is subject to selective screening in your journal or community. They are screen_set, screen_unset, and screen_list and they are very similar to the related console commands for banning users. The maximum number of users you are allowed to apply selective screening to is 500 per journal/community.
When a user is subject to selective screening, all of that user's comments in the affected journal/community will be automatically screened, regardless of what the entry's screening settings are. The affected user is informed that their comments are being screened when viewing the reply form. The user's screened comments are like any other screened comments and can be manually unscreened, receive screened replies, etc.
This feature was requested back in 2012 and I am excited that it has been implemented!