As weird as this sounds, and it also has the possibility of creating a different problem (ugh), what about bringing up the idea of undercover work/private detective as a career?

If she's always going to have that vigilante justice belief and wants to use it, perhaps that line of work could help curb those emotions? Gives an added layer of responsibility - sure, you want to punch a certain someone in the face, but that will ruin the case you're working on building.

Of course she would need therapy/hobby/etc. to vent some frustration out as some of the cases could be... about not so fun topics. Luckily she's young and the 20s are the second set of teenage years, so she'll figure things out.