You can remove the unwanted responses using the option available in the
​Results tab, then Responses. Select the three dots button in the upper right corner.

Then the Delete the responses... button will appear.

A message will appear asking you to confirm the deletion of the responses. You can decide whether you want to remove only completed or all responses.

If you want to delete a single response, there is also the Remove sheet option in the same tab. You will find it at the very bottom of the screen.