Purges in Cambodia might have changed average genotypes because they hit such a high fraction of the population. Generally it's hard to change things much in one generation, though - particularly because of loose correlations between genotypes and dreadful political fates. In the future dictators should be better at this. Now if Stalin had taken all the smartest people in the Soviet Union and forcibly paired them up, artificially inflating assortative mating for intelligence, you would have seen an effect. If you were a billionaire, you could maybe bribe people into something similar.