If you no longer want to use a piece of content in a stream you can either remove it or archive it.  Here's how to archive content.

Removing destroys all the history associated; archiving preserves it.

Archive Stream Content

Select your engagement program and go to the Streams tab.

Hover over the email you want to archive, then under the gear icon click Archive.

That's it! Remember, archive if you want to preserve history.

Unarchive Stream Content

Select your engagement program and go to the Streams tab.

Click on the gear icon for your stream then click on Show archived content.

Now that you can see the archived content, click the gear icon for the content you want to unarchive, then click Unarchive.

Perfect! Now this content is available to be prioritized and activated.