To turn off Ctrl-J jetways in third-party scenery, there are two approaches:
1 - if you're lucky, you may find one or more dedicated bgl files with just the jetways in them. You can rename these files from .bgl to .off.
2 - the GSX way, as illustrated in "Tutorial - Mass-replace jetways on a 3rd party airport in less than 4 minutes", is to use the GSX editor, (you park at a gate with a default jetway, you use GSX to customize the position, and you check the box for ajetway exclude radius of 1.0 meters. You then repeat this process for every gate with a default jetway (usually a /J) and then save your changes. Then you close the sim down, go to the GSX control panel, and compile your exclude file. When you restart the sim, you should find no jetways visible in the airport, although you may still see the /j associated with the gates. You can now add a GSX jetway and you'll get a /js designation.
Have you tried the GSX path? You've gone into GSX, edited a parking position, and checked the box for exclusion?, then exited the Sim, gone into the GSX app, and compiled the new exclude BGL file, then restarted the sim?