For a Mendix 4 example you can look at https://forum.mendix.com/questions/1933/How-would-i-logout-of-mendix-via-a-microflow
To get that to work, you can just create a Java action (returning a Boolean) of your own and copy/paste the part in between the BEGIN USER CODE and END USER CODE in the appropriate place, so it would look like this:
// BEGIN USER CODE
com.mendix.core.Core.logout(getContext().getSession());
return true;
// END USER CODE
In Mendix 5 you can even send the client a notification that the session is logged out, so it immediately resets the client instead of having to click around in order to get the client to notice the logout. For this you could add the following line of code
com.mendix.webui.FeedbackHelper.addLogoutFeedback(getContext());
(this should be on the line before the
return true;
statement)
com.mendix.core.Core.logout(getContext().getSession());
return true;