The loading range of JFK has been carefully selected to be made exactly in a way that, when you are at EWR or LGA, JFK is not loaded.
If you want to do the opposite, and remove EWR or LGA when you are at JFK, that's a bit more tricky and I suggest not doing it, because they are included in a file that contains several airports and navaids in the area, not just the airport.
You might try creating a separate exclude file with ADE or AFX, and be careful not to remove stock navaids.
However, if you are so short on VAS that you believe you might need to exclude a default airport, I suggest to look at other places first to save VAS, because you won't say THAT much excluding default airports (they are made using lots of stock library objects).
