Traffic Tools by Lee Swordy is an excellent tool, it decompiles the existing traffic bgl into a notepad text format, and after you make traffic changes or additions to it(the notepad text) save and then it is recompiled as a bgl file again.
Very easy, there's a manual goes with it so make sure you read it all up to get a grip on it.
WOAI traffic is excellent also, its self installing. They have some GA too also they have lots of US airline packages too.
Also there is the HTAI Cessna AI package, its payware but very worthwhile. Adds more GA AI to the FS skies and airports.
Before installing all of the above, you may disable the existing default traffic bgl, therefore removing all the default AI traffic.
I think there are redone traffic bgls you can download somewhere from AVsim perhaps or somewhere else I'm not sure, that only have the default GA without the default airliners.