Shouldnt we prevent some people from reproducing.?

Then what? The super strong would have to be even stronger, the super intelligent would need to be more intelligent. The world would be likely to be dominated by logic and little compassion. And intelligence has limits. What would become of great literature or music if you need to be "intelligent" at everything other than your expert field? What about those on a "lower level" that serve you at shops, clear your rubbish and clean your workplace? Heirarchy is as important as elitism.