If you are confronted with an instance of copyright infringement, there are right and wrong ways handle it.
If you encounter copyright infringement, here is an appropriate way to handle it:
-Report the instance of copyright infringement to the appropriate party
-(optional) Inform the offender that you suspect infringement and have notified the appropriate party
-Go about your business and let the involved parties sort things out.
-(optional) Advise others not to patronize the works of the infringer
DO NOT ENCOURAGE HARASSMENT OF THE INDIVIDUAL.
Seriously. Don’t do it. If Hasbro & Wizards of the Coast want to deal with someone writing bad Mystara fan fiction and passing it off as their own, they will deal with it, and I’m sure they appreciate being informed about it. But what’s bad is when a deluge of people feel the need to shame this person, harass them and encourage other people to do the same. This person has made a mistake by passing off someone else’s work as their own, and they’re going to pay the price, in terms of money and credibility. Their life is probably going to suck a lot in near future, so don’t make their life worse by posting harassing messages on social media or sending tons of emails to this person. Unless you bought something from this person or were someone who had their IP infringed upon, you have no reason to be in contact with this person other than your own satisfaction by getting your digs in.
Internet mob justice is NOT the answer.