Turn off Keep playing your Timeline, and keep Up Next empty.
When an episode ends, Castria plays the next one in Up Next. When Up Next is empty, it carries on with the next episode in your Timeline. Those are two separate things.
- To stop it continuing into your Timeline: open the Up Next tab and turn off Keep playing your Timeline, the last row of the list (when Up Next is empty, it's in the empty-queue card). You can also go to Settings › Playback and turn off Keep playing your Timeline when Up Next ends. It's on by default, and you set it separately on each device.
- To stop it playing your queue: clear Up Next (see How do I reorder, shuffle or clear Up Next?). Episodes in Up Next always play in order.
To stop after a single episode, set the sleep timer to End of episode. See How do I set a sleep timer?