If you repalce all of your default AI with the same aircraft you will have a huge problem as FS does not like duplicate AI aircraft. It is hard to have the same aircraft in more than one place at a time. You can have several traffic files but you have to make sure you have no duplicate entries.
One way to get rid of the default AI is to just delete it the other way is to go to projectai and download there uninstaller/installer.
You will find your default AI traffic in the Scenery/World/Scenery folder almost at the very end of the file it is easy to spot as it is some number like 60839traffic.bgl or something like that. 8)

Gee I thought that is what I said, granted I did not know the name of the ftraffic file but !!!!
Why repeat it ?
He was talking about default traffic not PAI traffic so why mention the file.
And if I am not mistaken if you only have the default AI installed removing the traffic file will make all AI traffic dissapear without any problems. Of course you can also just turn off AI in your setup menu as well and do the same thing but the actual traffic file would still be on your system unless you remove it. but leaving it there will not effect FS at all.
It depends on what he wants.
If you want no ai what so ever than just turn it off and be done with it. but if you want to get rid of the default and add some other kind of traffic than you will need to remove or edit (YOUR CHOICE) the origanl traffic file. 8)