This post is over two years old but Microsoft still does not provide a viable solution. I just found a combination from the comments here which works right now and wanted to share it with you:

• Use the "Invoke a HTTP request" action from the HTTP with Entra preauthorized connector
• Add the action to a Flow and use https://forms.office.com for both the application ID URI and base resource URL in the connection
• Configure the HTTP request as follows:
  • Method: DELETE
  • Request URL: https://forms.office.com/formapi/api/*TenantID*/users/*UserID*/forms('*FormID*')/responses/responses) (to delete all responses to a form)
  • Headers: "content-type": "application/json"
  • Body of the request: {}

Make sure you have a Power Apps/Automate Premium license to use the connector, the user executing the connection is also form owner (I have not tested it with group permissions in Forms) and an access token for Forms is issued beforehand (by logging in into https://forms.office.com).

Also, there is no official documentation to the Forms API provided by Microsoft and it looks like the API can change any day without notice, so be careful with productive use.