To have a secure log out for one or more applications which are using the centralized SURFconext authentication service you must close or restart your browser.
When you log in with you institutional account to one service which is connected to the SURFconext authentication service you will automatically log in to other services connected to SURFconext when you visit the URL of this service. This is called single sign on and this can be very useful.
There is a setback: When you log out from one service the single sign on can prevent a 100% secure log out for this service. To be sure the log out is secure, you should close or restart your browser.