What can be done to prevent mass murders such as the recent church assault? Nothing,

What can be done to prevent mass murders such as the recent church assault?  Nothing. 

Before the atheist moron killer entered that Texas church,  he had been kicked out of the Air Force with a Dishonorable Discharge and, later, denied a right-to-carry gun permit.  Turns out,  because of the Dishonorable Discharge,  this murderer was not allowed to own a gun,  much less carry one,  nor was he allowed to have guns in any dwelling he occupied.  In other words,  there are no laws that would have prevented  this mad man's ability to possess a weapon.  None.  

So stop with the talk i.e.,  "Surely we must at least try."  No point in "trying" in face of the fact that nothing,  absolutely nothing,  can be done.