I found an unknown device on my LAN. How can I remove it?

Network Analyzer cannot be used to disconnect any device from your LAN (it would be a really big security risk if an arbitrary app could do such a thing — think about all the fun one could have disconnecting others from wifi at Starbucks).

You need to check your router's settings — routers usually have a web user interface accessible at the router's IP address (in the case of Airport devices, there's an iOS or macOS app for that). There is usually a MAC address filtering feature where you can specify MAC addresses of devices that shouldn't connect to the network. Please refer to your router's documentation for more info.

Note that Network Analyzer also finds devices connected by cable to your router, which could be the "unknown device". And often you realize you simply forgot about some smart device, such as a thermometer, light bulb, or TV set, which can all be connected to the Internet. So be careful when disallowing a device from connecting to the wifi.