If your SpiderOak device is stolen or lost, to keep someone else from seeing data in your SpiderOak account, you need to do these two steps:
Log in to your account on the web interface and de-authorize the stolen device. This will prevent it from receiving any further updates from the server of any kind, and will cause the server to disallow future storage connections from the device. To do this, click on the "Devices" button in the navigation bar along the top of the web storage page. Find the stolen device in the list and click "deauthorize".
Change your SpiderOak password. This can be accomplished by going to "Account" within the SpiderOak application.