It cannot really be stopped. Too many guns, and anything can be used as a weapon. Firearms can be homemade,knives.  machetes.bombs sticks, poisons, fire. there are too many other weapons to say taking away guns is   THE   answer.
There are over 300,000,000 souls in the United States. If .000001 % are evil minded . intent on mass casualty incidents, that means 300 people fit the category. That is one millionth  of 1 percent. 
Steps can help slow it down by focusing on mental health problems, but it cannot be stopped.