[Lorenz-Kraft]

Well, I don't want to be rude, but a little google for "instagram report user" will quickly reveal https://www.facebook.com/help/instagram/165828726894770

If you in "general" asking about online harassment, that's another topic. I think then everybody just misunderstood your original post (which seem to reference to Instagram specifically).

[throwaway78961]

Maybe, you can also help by reporting the person for spam or inappropriateness? Not too much to ask for, right?