As long as someone has an opinion, true or untrue, it's allowed. They can't just say, "Boxman90 is a cocksucking bitch" if that's the entire post. However, if they say, "Boxman90 is an immature donkey and should be put in a zoo. He is incompetent in his running of Mt. Cox and is not fit to run even a mommy blog, let alone an exchange processing millions of dollars. Heck, even password resets are sent via plaintext, like a test website for a retarded monkey taking middle school Python."
There's really not much you can do except provide one long reply with a good, complete argument for why you have done nothing wrong. Spend some time, make that post, and leave the discussion. If anyone asks you about it, just point them to that post.
The one exception would be if you do file a lawsuit against this member and the court requests that it be taken down. While I don't think that has happened before here, I assume the mods would just take down a defamatory post to avoid the legal issues of not complying.
This is misleading. Anyone is able to give (post) their opinion however they need to give an opinion and not present something as fact that is not fact.
An example of an opinion is "I think...." or "I would speculate that..." or something similar.
In order to be slander, something needs to be presented as fact, be untrue, believed to be potentially true by a reasonable person, and reasonably hurt your reputation.
I do not think there is precedent of slander of an online identity that is not your "real" identity. I do not think that slander exemptions to freedom of speech would extend to an online identity. Even if you are not conducting business the court would likely find that an online identity is similar to a business name which is not subject to slander protections.
I also do not know of a case in that courts have ordered false information to be taken down from a public forum, even after the person who posted something false has been found liable for slander. The supreme court has found that websites that host content provided by others is not responsible for what it's users post.
In regards to someone being called a criminal, this would almost never be considered slander because everyone has likely broken the law in some way (for example because of speeding - even if you are not found guilty in court of a crime does not mean you did not break the law - if you have broken the law then you are a criminal).
Being called a scammer is also very tricky (especially because most of the scam accusations generally use some kind of opinion, or include the term 'likely') as there is not as clear of a definition of the term scammer and what one person would consider to be a scam could potentially be considered to be legit by someone else.
I do not believe there is a posted policy regarding taking down slander, however I would think the forum would allow it as long as it is not obvious trolling. The forum has a policy against moderating scams, so if they were to moderate scam accusations then they would essentially be endorsing whatever it is that you are doing and in effect saying that you are not scamming when it is not 100% sure that you are not going to scam in the future.