If we do not want to give up our guns, how do we prevent mass murders like the Arizona massacre?

This is an outgrowth from another thread. Some people are defending their right/need to carry guns. Fine – but if we cannot get rid of or even seemingly control gun purchases, how do we stop the wingnuts from massacres? Yes, a nut can get a knife or bow and arrow, but guns (and bombs) are the most efficient killing machine. (Two questions in one day? It’s a snow day and I’ve got lots of time to Fluther.)