Well, without more context it's hard to say exactly. It could be that in the fan community surrounding Jamie Tartt, there have been instances where fanfiction has been treated poorly. Perhaps some people have made fun of or shamed certain types of fanfiction, or there could be a case where the rights to fanfiction have been violated in some way, like if someone tried to claim the fanfiction as their own original work when it was clearly based on the character Jamie Tartt.
I'm not sure specifically about 'Jamie Tartt abused fanfiction'. It could be a very specific claim within a particular fandom. Maybe it involves some unethical or inappropriate use of fan - created works related to the character Jamie Tartt in a way that is considered 'abuse' like misappropriating, plagiarizing, or using it in a derogatory context without permission.
There could be several interpretations. If 'Jamie Tartt' is a character from a show or a story, 'abused fanfiction' might mean that someone has taken the fan - made stories about him and either distorted them in a negative way, or used them in a way that goes against the original intentions of the fans who created the fanfiction. For example, if a company took fanfiction about Jamie Tartt and made it into something completely different and inappropriate without the consent of the fan authors.