I was thinking three enormous ones might cut it. Certainly the government would regard them as reason enough. Then the government doesn't want superpowers running about at all. Too hard to corral -- and those that aren't corralled can only be managed by those who have, giving them more bargaining power. But the general public? Who feature as those to gain? Three sufficiently ugly and large-scale consequences might quell them enough they can be kept back.
Plus, possibly, stories of people going mad. I think that will be all story: the "mad" are those whose vicious tendencies were curbed only by threat of force before. But maybe not.
And perhaps the best would be the way it knocks you down for a week or a month like a bad case of flu -- and then there is no guarantee of powers. That's probably enough for all but the thrill-seekers. (Hmm -- imagine the resentment of a bunch of thrill-seekers where only one gets it. . . .)