Can A layer-4 firewall block TCP traffic from a specific user on a multi-user system?

Question

Layer 4 firewall is a device that can look at all protocol headers up to the transport layer and not the headers above that layer.

Answer 1

No, it can't. There's no reliable way to identify the user at the Ethernet, IP or TCP levels, which is all it has to work with.

Answer 2

You can firewall on a per-user basis only when firewall is running on the same OS instance the application the user is running.