The Filter tab on the per-channel Podcast Settings dialog can be used to block all episodes or specific episodes for a channel. Blocking marks episodes as old, preventing automatic download.
Selecting Block and leaving its text field empty will block all episodes for the channel. Entering text performs a partial case-sensitive match on the episode title. The Ignore Case checkbox performs a case-insensitive match and the Regular Expression checkbox treats the text field as a regular expression, enabling complex matches.
Selecting Except is used to override blocking for certain types of episodes. The most common case would be when blocking all episodes, and only unblocking the ones that match a pattern. A specific category of episodes could also be blocked and this would unblock episodes in the category that contain a keyword.
It is not recommended to select Except and leave its text field empty, as this effectively disables all blocking.
The Filter episodes now button can be used to test changes to filters by applying them immediately, without needing to update the channel.