In the Jottacloud software go to the "Restore" tab, and select "Links" from the list. You will now see a list of all the public url's in your Jottacloud account.
Click "Unshare" to make an url private again. After doing this the file won't be shared any more, and it cannot be used to download the file the url pointed to.
You can do the same when you are logged into www.jottacloud.com. Click "Links" in the left hand menu. To stop sharing a file, click the little crossed out chain icon to the right of the file name.