Once you create characters, you don't always have the control over them you might like. That's life.
This was the case for me.
I agree with these sentiments and with Marina's comment that Jeff quoted. I've never written fanfic, but I've read a ton of it.
I feel like I've said this a lot before, but I guess it is worth repeating. Things that are equivalents to fanfic happen in culture all the time. Artists and writers borrow ideas, tweek compositions and quote one another all the time in all kinds of different ways. Parody (both very serious and lighthearted) have been a part of literature, theater, art, etc. for ages.
When you think about it, the movie of Brokeback Mountain is already a kind of "fanfic". this echos some of the ideas brought up by Marina in her post. The movie includes many elements and embellishments that do not exist in the short story. For instance the SNIT is something invented beyond the control of Proulx... as with the details about the female characters, a million elements of dialogue, etc. The screenplay was written by two other authors... and it's filled with ideas from McMurtry and Ossana... separate, again, from Proulx's control. I don't think many folks here have much of a problem with the idea that McMurtry and Ossana (and Ang Lee, the actors, etc.) altered BBM in order to make it into a movie.
People have been writing novels based on Jane Austen's novels and characters for years. And, people have made alternate versions of Shakespeare's plays (sometimes radically altered) for years too. And, then there are things like Wicked... which, seems to be a very high-level example of a type of "fanfic." Issues of appropriation are basic aspects of a lot of English courses these days. And, a lot of high level literary theory deals with issues of appropriation because it's fundamental to how ideas in art circulate.
What Annie may not understand, because I doubt she's probably actually read a lot of the very good and thoughtful fanfic.... is that many of the good fanfic stories are very respectful, introspective, etc. And many are so different from the original BBM that they really can be said to be only tangentially inspired by BBM. There is definitely a lot of bad and/or silly fanfic out there, but there's a lot of very smart writing out there too.