The best way is to do some Google stalking and find out where they live, then explain to their face what you are planning to write in the forum. If they call the police or punch you in the face, don't post it.It's just so ambiguous though!
On one hand, I'm expected to be a decent person, state my negative feedback politely, be considerate for others, feel free to argue but do it respectfully and so generally be a decent human being and on the other hand I'm expected to not target someone personally and abuse them, harrass them or attack them directly or wish them or their families harm. How can I tell the difference???
Yes, sorry... very sarcastic. Sorry!
There, a simple solution which works for everyone.