disable NIC for a local user only

You need to back up and rethink the goal. The goal is that you want one user to have limited network access. Your solution right now is to disable the network interface for that one user. Instead, you can just limit their network access by limiting their network access. Do you use any kind of network management like a UTM/gateway that authenticates users and allows them to access internet? At some point, you'll have to explain your environment and what you are trying to avoid so a more accurate suggestion can be provided.