You can either remove or archive a piece of content. Unlike removing stream content, archiving preserves all the history associated to the content. If you don't mind losing the historical stats of some content and want to remove it, here's how to do it.
Go to Marketing Activities.
Select your engagement program, then click on the Streams tab.
Hover over the content you want to remove, click on the gear icon when it appears, and click Remove.
That's it! Now you know how to remove a piece of content.