The next time it won't let you disconnect, try to left-click on the net_applet icon (will start network center), select the active interface, there is a button on the right bottom of the window, labeled "Disconnect/Connect", try to use that button, it could be a temporary workaround until a solution is found.Sorry, I left that out that I try to do that and it does not disconnect that way.
That could exclude net_applet as source of the problem, not sure about, need to look into ...
Another possible workaround, this time form a terminal, it to use one of the following commands, after su
service network stop
service network start
service network restart
the meaning of each one is obvious.