there are two ways to achieve that, depending on the scenery
1. Mark all the gates in GSX airport configuration and tick the jetway exclude box. Most of the time the default 1 meter exclusion radius will be sufficient to cover all the jetways, but you may have to adjust this setting, especially for gates with more than one jetway. I had to use 3-5 meters for some gates to make sure all default jetways are being excluded. Dont forget to re-compile the exclude BGL after you set the exclude radius in the configurator
2. Disable the BGL that contains jetways directly in the scenery folder of the addon airport. Depending on how the naming convention of the BGL files it may be easy to identify. Just rename that file from .BGL to .OFF and this should get rid of all default jetways (caution, some sceneries have two BGLs for jetways. One for animated default jetways and one for static jetways)
While i prefer option 2, i suggest to go for option 1 if you dont want to fiddle around with scenery files. Option 1 is non-destructive and wont touch any scenery files at all