I have uninstalled all the airports (CYVR, KDFW, KJFK, KLAX, KORD, PHNL and GSX. I have deleted all the associated folders. Still showing in the scenery library.
Something's wrong. I assure you our installer removes and disables the scenery.
It does it by using a special feature that P3D4/5 have, in which you can call the simulator from the command line as it was an utility, so you don't see starting it up, but it will add or remove an installed scenery. This is the LM-sanctioned method to install/uninstall a scenery, and should always work.
It might not work if, for some reason, our installer wasn't able to execute prepar3d.exe, possibly because antivirus interference, but I must say this hasn't been reported before.
In any case, removing the scenery folder from inside the Documents\Prepar3d addons folder will surely remove the scenery.