Last updated: April 2, 2018.
This article details what action you should take if your Instagram account has been stolen. Although this is likely rare, it can happen (from disgruntled former partners, associates, employees, and the like).
Please note that this article is not advising how to obtain a username that someone else is using.
This article is solely advising on having Instagram delete an account that falsely represents you and your name.
For this to be sucessful, the account should be in your name, have a picture of you, and you need to be willing to take a picture of yourself holding ID (to be privately sent to Instagram).
Be certain you can't access the account
Before proceeding to reporting the account to Instagram, be sure you can't simply change the password.
Attempt to reset your password.
If you have access to the email address associated with the account, you can reset your password using the link in the password reset email. Also ensure you secure your email service, and (if necessary) change the email address the account uses - to prevent the account from being stolen again.
Report the account as an impersonation
Instagram will delete properly reported impersonations.
The justification for reporting the account as an impersonation is that, although you may have created (and once controlled) the account - someone else is now in control without your permission.
Take a picture of yourself holding ID
Get someone to take a clear close-up photo of yourself holding acceptable ID.
Instagram accepts a wide array of ID, but the most official identification you have is best (an internationally recognised passport would be best, for example). You can cover any private information, so long as it shows your name and face clearly.
Instagram prefers the JPEG image format.
Submit a report
Submit a report via the Report an Impersonation Account on Instagram form, selecting either "Someone created an account pretending to be me or a friend" (if you're reporting for yourself) or "Someone created an account pretending to be someone I represent (ex: my child)" (if you're representing someone).
Enter your full name (as shown on your ID), and your email address (an address only you have access to).
If you're representing someone, state your relation to them. If the relation is parent (for a minor), state parent. If the relation is legal, state lawyer or legal representative. Otherwise, simply state authorized representative - this indicates that you (with an irrelevant relation) have the authorisation from the person you're representing to submit the report on their behalf. Do not submit a report without this authorisation.
Fill out the remainder of the form, uploading your photo (with ID) and entering the username of the offending account.
Under description, simply and clearly state that the account is unauthorised. Instagram is uninterested in backstory and personal dispute.
Below is an example.
I have not authorised any party to create or operate the reported account, nor do I have access to the reported account.
Below is an example if you're representing someone.
The individal I am representing has not authorised any party to create or operate the reported account, and have stated that they do not have access to the reported account.
Submit the report. Instagram should delete the account in a number of hours. It's best not to interact with the stolen account, or to encourage others to do so.