All properly run forums are moderated, and can remove posts that are libellous or slanderous or defamatory.
"Free speech" is not a licence to make untrue statements that cannot be backed by evidence and proof.
If you make a statement about another person on an Internet forum, this is no different to making that statement in print, on radio or TV, or in a public speech.
If you make such a statement declaring a named person to be of doubtful character - or that named person is guilty of chargeable offences and hasn't been charged - then you had better be very well-prepared to back your public statements in a court of law.