There are a number of ways, I'm not inclined toward authoritarian measures. An incentive structure could be built to encourage genetic interventions (at least once that technology matures). Just kind of like some natalists want to encourage births in general. It would have a pretty gentle effect at most but again, I'm aiming for overall harm reduction. The reduction of male births isn't the goal, its a tool to reach a goal.

In addition, a cultural movement could apply soft social pressure and normalize the practice of aborting boys and trying until a couple gets a girl. This would be difficult though not impossible. Cultural movements that started as a minority opinion pop up pretty frequently these days.

If we were to get authoritarian with it though, of course things get both easier and harder. Easier in that you could just mandate things, in reality this would probably just create black market situations and cause more harm than good. It might be possible to put something in drinking water to significantly decrease the chances of male births as well if you want to validate the conspiracy theorist types, though you could just be open with it and get people to accept it.