One way to force logout is closing the browser (with all its windows) but that's way too radical.
Fortunately visiting the a link like this should also work in most cases.
You can type it every time to the URL bar, or create a bookmark with this in the URL field:
What it does is simply replace the
://logout@ in the document url, so it works with both http and https sites.
Now you just need to click on the bookmark and you'll be logged out.