Sometimes there's a riff that you don't really plan on watching again anytime soon. (*cough*AnythingbyMichaelBay*cough*)
For that reason, we've introduced the "Archive" to where you can banish riffs, and have them not appear in your Library. Just click the button to Archive them, or click it again to move a riff out of the Archive.
Use the "Show archived riffs" checkbox in the Library filter sidebar to see your Archived items at any time.