Dungeon Defenders II cannot be shared. To disable Family Sharing, log into the Steam client and visit the "Family" tab in Settings, where you'll find a button to "Manage Other Computers..." Here you can deauthorize any computer or account that's been authorized to share your games. You can also remotely deauthorize devices from the web. To do this visit your Account Details online, and then select "Manage Family Library Sharing". Using either the client or the web site, you can view all currently authorized devices and accounts and disable any or all of them.