There is no "active" or "enabled" property for the user by default. Here's what you could do:
If you created custom roles in your application, you could just override the user model and add an "isActive" property, then check if is active when the role is called
Alternatively, create a middleware after authentication. Loopback authenticates the user, then you catch the reply and do a 2nd authentication to see if the current user is active or not. return an error if not.
Hope that helps
F