Bringing the company into disrepute because of a post I can understand.
But being disciplined because you call your manager an incompetent fool.
C'mon thats taking it a bit too far.
I assume that you live in a country that may be considered a democracy. That being the case, you can call your manager whatever you like.
Just have the guts to do it to his or her face, and do not hide behind the so-called "protective screen" of the internet. Or, put a different way, be prepared to accept responsibility for your actions.