Gooner, this is exactly the rule already existent. According to the rule, athletes representing a Country and after changing citizenship are automatically allowed to compete for the new Country after 3 years from the date of the last time they competed in an institutional Championship for the previous Country. This period can be shortened to one year, but in this case the athlete needs the "release" from the old Country.
This is what happened with Wilson Kipketer (who won the WCh in 1995 and 1997 but was not allowed to compete in Olympic Games 1996) and Stephen Cherono alias Shaheen (WCh 2003-2005 thanks to the release of AK, but not allowed to compete in Athens OG 2004 because, for Olympics, the athlete has to have a specific release not from the previous Athletic Federation, but from the National Olympic Committee).
Trying to have more control about the change of citizenship, some year ago IAAF changed the rule, putting as "must" the point that athletes changing nationality had to demonstrate to be resident in the new Country for longer than 3 years.
The fact is that this rule never is followed. Some of the Countries using to take athletes from other Countries (Turkey doesn't look at African and middle distances only : they have also the sprinters Guliyev (19.88) from Azerbaijan, Harvey (9.92) from Jamaica and the European Champion of 400m hs Copello (47.92) from Cuba) have a "legal" system that is a trick : the athletes go to compete for some National Club in the Club Championships, staying in the new Country for, maybe, two weeks in one year, but the Clubs give them the yearly Visa as resident, so, when they have 3 years of "fake residence" they can officially change citizenship, according to the current rules.
The fact is that there is no way to stop athletes from competing for a new Country, if they didn't represent in official way the old Country. If I am a normal Italian citizen who wants to become, for example, Turkish, at the moment I have the new citizenship I can obviously compete for my new Country, and the ball is in the hands of the new Country, that can decide how long you have to wait for the new citizenship.
This is the case of Bahrein, that has all athletes never representing in official international championships their previous Country (Kenya or Ethiopia), so there is no legal way to stop them.