There's already been threads on this issue in the past, such as:
http://www.dbocom.com/forum/showthread.php?t=26562
Depending on the type of event, evidence might be impossible to verify with 100% certainty and possible to fake. Even if this wasn't the case, I still wouldn't allow it because I see no value in calling people out. Lists of scammers only provide a false sense of security, as lists will always be incomplete and scammers have ways to hide their identity. If this is about helping people, then you should let them know how to safeguard themselves against the methods used by scammers & hackers.
Back when there was no rule against this, it also was never as simple as "this guy's a scammer, lets all outcast him", as the scamming tended to be a part of conflicts between groups. When someone was called out, that person's friends would defend them and justify his actions by claiming they themselves were somehow previously wronged. In some instances, the person being called out enjoyed the notoriety they received from having a thread devoted to them.