They can't stop bitcoin. They can certainly stop the exchanges. They're registered businesses, linked to banks and they're run by known people. A few phone calls and visits and the party stops.
Thats enough to effect the market lol.
Which doesnt help people to get involved - people get involved when they see profit. So if they dont see it in the exchanges they kill bitcoin indirectly or delay the growth of it..
Bitcoin may not die, but its growth is from exchanges.