The same way that Bush 41 did, right about the time of his tantrum over the NRA's use of the phrase "jack-booted thugs" to describe BATF.
The '68 Gun Control Act gives ATF import/export oversight with regard to guns, ammo and certain accessories. The President really doesn't even need an executive order to do this (though Obama will use one to score political points with his supporters). The controls are already in the law, all he needs to do is call the BATF(E) administrator and direct him to exert the agency's authority in such a way to accomplish his goals.
If this is correct (and Rawles is usually fairly reliable), we've seen the last of the cheap 7.62x39 ammo and AK magazines (just two obvious examples). There might be some shipments already stateside that haven't made it out to retailers yet, but the prices sure won't be going anywhere but up.
Don't forget Bill Bennett was in on the gun import ban too.