You can delete single episodes from your device and redownload them at a later date assuming you are in a country that supports iTunes in the Cloud.
The table below shows you what types of purchases you can download again in each country. If a certain type of content can be downloaded again in that country, a ✓ will appear in the country's row for that content type. If music, apps, or books are available in your country, you can enable Automatic Downloads on your computers and iOS devices to automatically download new purchases made from other devices. This feature is only available for music, apps, and books.
Download past purchases
When you delete an episode from your device a cloud button will appear next to it. You can use that button to redownload the episode at any time.