Is there a way to make an 100% self contained league system?

Hello! 
 

I tried to search for an answer to this question, but haven’t found any answers anywhere, so here I am

I am in the middle of creating an 100% fictional database, made up of 5 fictional nations, with league systems for all 5. Because this world is totally fictional, I was wondering if it’s possible to prevent players from jointing any “IRL” leagues, as well as preventing my fictional teams from signing any “IRL” players. Basically, I want these 5 nations to be entirely self contained with no input from the “outside world” 

 So far I have made a rule where 0 forginers are allowed, and made it so every one of the 5 nations acknowledges each other’s players to be “non foreign“. This works for the most part, but every year there are still a few players from IRL nations that slip through the cracks (even though they just sit on the bench and never play)

As far as I know, you can’t just mass delete every other league in the world. So is there a better way to do this than my current system, or am I just out of luck?

Thanks for any help!!