How can I stop my brother from using ARP Spoofing tools like NetCut on our network?

If he isn't paying for the internet, don’t hesitate to kick him off! Change the router's admin password and remove his access completely. Let him know that this behavior won't be tolerated.

A more advanced option would be using a managed switch to keep track of MAC addresses and effectively block unwanted devices. This does require some extra equipment and setup, but it could solve the problem long-term.

If he's using NetCut, you might not be able to access the router. In that case, try to take away his internet access outright—whether by physically disconnecting the cable or changing the WiFi password if he's on wireless.

Consider setting up your devices with static IP addresses. This way, you can keep track of what's on your network and easily identify new devices he might connect. This setup might create some extra work initially, but it could save you a lot of trouble later.

One simple way to address this is by blocking your brother's computer directly from the router. Just go into the router's security settings and you should see options to manage or block active devices.