In the TotalScore page, after deploying the patch, instructor will have the ability to delete any of the students' submission.
This feature has been initially created in Texas State University by Zach Thomas a few years ago. I later on improved the feature and fixed some issues. It's been working pretty stable in our production 2.6.2 version.
I've created a patch for trunk at revision 85614 and tested briefly, seems working fine. So when a test submission attempt is deleted, it will also update grade at gradebook if this test is report to gradebook.