Depends on how it was installed. If it was installed through Extension Manager, open Extnsion Manager and find it, then click Remove. If it wasn't installed through EM, just find it in the application plugins and delete it.
I found the same problem - i had to go to do the following:
delete "/Applications/Adobe InDesign CC/Scripts/the folder containing the extension you wish to remove"
Make sure you have removed it in the Manager as well
Extensions and plugins are no scripts.
Because the Extension Manager is to be discontinued you can delete, remove or rename with a non-valid extensions installed plugins to uninstall them.