There's two problems I see with this - one with an inadequate understanding of evolution, and one with an inadequate understanding of how being 'smart' works.
First of all, about the evolution thing:
We haven't eliminated natural selection. Natural selection is when life that can't survive in the current environment is snuffed out, halting the 'bad' genes. The ones that remain are the ones strong enough to survive.
We haven't 'reversed' or 'halted' natural selection at all. We've only changed one thing: the environment. Our environment is now much easier to survive in. Natural selection still occurs in humans - just at a much lesser level. People with horrible diseases still die, and people who have a ridiculously low IQ usually don't reproduce.
Maybe eventually our environment will 'strike back', so to speak, in the form of global warming or whatnot - but my point is this: those that live long enough to reproduce are fit to survive this world. There's simply a greater tolerance for stupidity thanks to an easier living environment. That doesn't mean people at the far low end of the spectrum won't be eradicated from the gene pool, though.
My next point is about the subject of IQ.
Regardless of the ethics of genocide, it won't work. By raising our IQ, our society will have a higher IQ on average. However, they won't be 'smarter' in the way we perceive it. There would simply be a new spectrum. Now, instead of IQ varying from 75 to 150 (I'm pulling these figures out of my ass, it's just an example), it would vary from, say, 100 to 175. People with an IQ of 100 would be perceived as stupid by the new society - after all, they're at the far low end of the IQ spectrum.
In addition, having a higher IQ wouldn't drastically help society. In fact, I would say it would be detrimental. More problems would be solved, yes - but far greater problems would be created. Smarter people tend to be more opinionated and more volatile. In addition, they tend to be slightly more crazy.
The stupid people are the ones willing to simply follow orders - the smart ones are [usually] the ones leading the revolution (or ensuring a new country's success). Our society, in my eyes, would become much more volatile. We'd solve a lot of problems, but we'd also realize we have a lot more to solve. As the saying goes, the more you know, the more you know you don't know.
Regardless of the ethics involved: It simply wouldn't work.