To manage your shared links visit Shared in your account settings. Here you'll see a list of all your shared links sorted by last created, whether they're currently Private or Public, and if Password Protection is enabled or not.
To stop sharing an item, click on the ••• menu button to the right of the shared item. Then select Edit share. Simply toggle the link from Public to Private and you'll see the state change to a grey "lock" — indicating the link is no longer available for viewing.
Note
If you'd like to be certain your shared link is now disabled simply copy the link into a New Incognito Window (or New Private Window) in your browser and see for yourself.
Alternatively, you can follow the same steps above but select Delete share if you'd like to never have the link accessible to anyone — this is recommended only if you don't plan to share the item ever again.