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 just want to remove it, here's how to do it.
1. Select your engagement program.
2. Click on the Streams tab.
3. Hover over the content you want to remove and click on its gear icon
4. Select Remove.
Remove content only if you don't care about its history. If you want to preserve its history, archive it instead.