You can use this form to report defamatory content to Instagram.
If your country of residence is eligible, select I am reporting on behalf of myself. Next, enter your information and give the URL of the account you want to report. In your report, provide as much details as possible to prove that the account is defamatory.
In case that account seems to be going against Instagram's Community Standards, such as bullying or harassment, impersonation, false information, etc. then try reporting the account directly from the profile page.
There are no posts. They have used the name, description and profile picture of the account to make defamatory statements about us. There is no other URL related to this besides the one for the account.
Try reaching the Oversight Board. You can contact them if you want to appeal the decision made by Instagram about the content you reported (or about your own content).
To check if you're eligible to appeal Instagram's decision to the Oversight Board, go to Menu > Settings and privacy > More info and support > Help > Support requests > Report. If you've an Oversight Board reference number next to your report, then you can contact them.
However, note that the Oversight Board only selects a number of appeals that are eligible for review. I can't guarantee that they'll look at your appeal.