Someone that receives a Share Link to a folder or file can forward that link to additional people.

Make sure you have set the Share Link Options correctly if you want the link to be only used once. Also consider adding an expiration date so the Share Link automatically becomes invalid on a certain date.

If you believe someone has forwarded the Share Link to others and you want to prevent access you can “Kill” the Share Link from all further use by returning to the Existing Share and clicking on the trashcan icon (Delete) for the Share Link created.  This will disable the share link.