MS Forms API implementation

Well, just in case you are still interested in how to delete responses from an specific user, here is an approach that I tried and it's working for me.

First of all, you need to understand the structure of the Forms API.

https://forms.office.com/formapi/api/[TenantID]/users/[UserID]/forms('[FormID]')/responses

[TenantID] - no doubts about it, tenant ID where the form is hosted.
[UserID] - is the UserID of the Form owner.
[FormID] - no doubts about this.

If you try this API and take a look to the JSON, you will see each response has an ID.

To delete a single ID, you just need to use the delete method specifying the response ID.

https://forms.office.com/formapi/api/[TenantID]/users/[UserID]/forms('[FormID]')/responses([ResponseID])

But if you need to delete all responses from specific user, you may use the first API call to collect all responses, parse the JSON, filter the array by responder property. Just note the responder property uses the responder UPN, not the email.

Now you have an array with the responder UPN and the response ID, just use an apply to each loop to delete each response.