UTM is forwarding NetBIOS?

Hey all,

I need an advice for pointing me in the right direction.
I discovered some NetBIOS packets in 2 of our local networks. These seems to be forwarded by our UTM. Because of the name I discovered the source of these. It's an NAS device in a foreign network. This network and ours are connected through a RED Tunnel. 

But why is UTM forwarding these? And more important, how to stop that? I tried firewall rules, but that didn't worked.

Best regards