To sign out of the control panel, just click Sign Out link in the top pane. You will be taken to DotNetPanel sign-in page.
Many browsers keep your session in memory even after you sign out. If you are not the only one that uses your computer, then be sure to close all running copies of your browser before leaving your workstation. This will clear the browser's memory and make it impossible to sign-in to the control panel without the correct password.