Bullying or abuse on Instagram can happen in many ways. It can be either negative comments, fake profiles or hacking of accounts. Instagram take all of these violations very seriously and have plenty of advice on their pages if you or someone you know is the person who is being abusive. However, if it continues or it has gotten worse, you can use their in-app reporting tool. This page has details on how to report the abuse directly to them.
It is so easy to be led into a false sense of security with these reprobates.