How so? There’s so much you can do to harass people within the limits of the law.

Just name and shame the individuals developing this kind of software, they deserve much worse but it’s a good start.

Vigilante justice will land you in jail. There's not a whole lot you can do from there.

Not at all, only very specific kinds of vigilante justice will land you in jail. It is not difficult to hurt people without breaking the law.