<<Other than putting everyone capable of violence (everyone) into fishbowls for lifetime observation , (which is what they are doing in London) , or restricting access to guns so harshly that Pol Pot would be embarrased by the measures , what could you actually do to make someone who is willing to plot his violence for months , helpless and frustrated to do so?>>
The obvious flaw in THAT argument is that everyone who starts out plotting violence doesn't last the whole month. Some of them will give up when they can't get the guns as easily as they thought they could. The ones who stay the course aren't the ones who the gun-control policy will stop. It stops the others, the ones who aren't so smart, resourceful or daring.