There is no logout button on the web interface. Instead, close and reopen the browser. We use basic auth for billing and account web pages, so the session is handled by the browser. The equivalent of logging out is to close the browser and wait for the session to expire, which will take place automatically in a few minutes.
Some users find closing the browser to be annoying, and we are sensitive to that, but it is important to realize that on any website, merely logging out or closing a tab or navigating away is insecure. This is not a SpiderOak One deficiency; it is just the way the web works. That is why many online banking services also recommend that you close the browser after finishing your session.
It is for this reason that we have resisted feature requests for a logout button on the web interface. It would be mere security theater and would leave you open to trivial attack unless you also closed the browser. And if you close the browser, then there is no need for a logout button.