DLNA doesn't support authentication.
You have a few options to deal with this.
1) You could turn DLNA server off if you don't need it. Tools->Options->Media Network->Advanced->(uncheck the DLNA server box).
2) If you use the DLNA server for Music but don't need images, in the DLNA server configuration Tools->Options->Media Network->Add or Configure DLNA servers->Customize views.
3) If you need more fine grained control for that you could probably use the options to limit the file display to files that aren't tagged with a custom field, user, etc.