If you don't need access to a storyboard any more, you can archive it and have it removed from your project dashboard. Archiving a storyboard also means that anyone you've shared the presentation mode URL with will no longer be able to access it.
To archive a storyboard, visit the storyboard page, hover over the three dots in the top right hand corner, then click "Archive storyboard" in the menu.
This will then open the archiving sidebar. If your storyboard has more than one version, you will see the option to delete only the current version by clicking the green button, or delete all versions by clicking the grey link underneath.
If you accidentally archive a storyboard, or want to restore an earlier archived storyboard, go to your project dashboard and click the "Archive" link in the top right hand corner. You'll then see a list of all your archived storyboards and be able to restore them.