Firebase how to unsubscribe

I am using Ionic2 with Angularfire2 to access Firebase Authentication.

I access the following rxjs/Observable:

chats.ts

  this.firelist = this.dataService.findChats();
  this.subscription = this.firelist.subscribe(chatItems => {
     ...
  });

  ngDestroy() {
    this.subscription.unsubscribe();
  }

dataService.ts

findChats(): Observable<any[]> {
        this.firebaseDataService.findChats().forEach(firebaseItems => {
           ...
           observer.next(mergedItems);
        });
}

firebaseDataService.ts

findChats(): Observable<any[]> { // populates the firelist
    return this.af.database.list('/chat/', {
        query: {
            orderByChild: 'negativtimestamp'
        }
    }).map(items => {
        const filtered = items.filter(
            item => (item.memberId1 === this.me.uid || item.memberId2 === this.me.uid)
        );
        return filtered;
    });
}

Also, in Firebase Authentication, I have the following Realtime Database Rules:

  "rules": {
      ".read": "auth != null",
      ".write": "auth != null",
  }

Problem

This works perfectly while a user is logged in (i.e. auth != null), but a soon as a user logs out, and auth == null, I get the following error:

ERROR Error: Uncaught (in promise): Error: permission_denied at /chat: Client doesn't have permission to access the desired data. Error: permission_denied at /chat: Client doesn't have permission to access the desired data.

Question

As you can see in the above code, I try to unsubscribe from the Firebase Authentication Service, but must be doing it incorrectly. If anyone can advise how I should unsubscribe in order to stop my app querying the Firebase database, I would appreciate the help.