I think Bellamy was totally in the wrong and although a fine or suspension would have been harsh, after all the fan should not have been on the pitch in the first place.
I do believe Bellamy should have a warning or some kind of punishment.
There was a time when the legendary Brian Clough slapped two fans who had invaded the pitch when he was the manager of Nottingham Forrest, after the incident the fans were so ashamed of ruining the day they actually apologised to Brian Clough.
However Bellamy walked up to this fan after 4 police officers were already holding him and restrained him and then slapped him in the face. It was not needed and totally out of order.
If it had been the case of the man was running at Bellamy and he (Bellamy) felt he was going to attack him then fair enough, but to do it when he has been restrained is not right.