There are three common ways to deprioritize or remove content you do not currently need anymore.
Deprioritize old documents
If you'd still like to keep the documents around in your knowledge base for reference, but don't want to practice their flashcards anymore, or want to practice them less often, you can reduce the documents' priority to Paused or Maintaining using the priority drop-down in the upper-right corner of the document:
If you set the priority on a folder, it will also apply to all the documents below it.
Read more about prioritizing documents in Setting Priorities and Disabling Flashcards.
Archive old documents
If you don't expect to need the documents on a regular basis anymore, and you don't want to see them in search or reference them from existing documents, you can move them into a different knowledge base.
First, create a new knowledge base, or decide on an existing one to move your content to. Then pick an approach:
Export the documents you want to move and import them, then delete them from the original knowledge base once you've confirmed this worked as you wanted.
Cut the documents, switch knowledge bases, and paste them into the new location.
Cutting and pasting is usually easier, but it doesn't give you the opportunity to see your changes in both the old and new locations before removing them from the old location, so it can sometimes be confusing with large numbers of documents.
Review Moving Content Between Knowledge Bases for details.
Delete old documents
We usually don't recommend completely deleting content just because it's old, because storage space is cheap and old notes can come in handy at the most unexpected times, but if you're sure you never want to see the documents again, you can delete them by selecting them and using the /delete omnibar command.