bannedUserRule.swift

service cloud.firestore {
  match /databases/{database}/documents {
  
    function isAuthenticated() {
      return request.auth != null;
    }

    function isBannedUser(userID) {
      return exists(/databases/$(database)/documents/bannedUser/$(userID));
    }
    
    match /comment/{commentID} {
        allow read: if isAuthenticated();
        allow write: if isAuthenticated() &&
                        !isBannedUser(request.resource.data.userID);
    }
    ...
}