Q&A for How to Disable Internet Access

Definitely! It depends on how advanced with technology you are though. If you know what 192.168.0.1 (or 192.168.1.1) is, then you know that you can access settings with your router. Inside there, you can find the device and add the device to the MAC Address blocking list. This will block all access to the internet. They can connect, but it won't let them use it at all. Whether that is available or not depends on your router, but anything fairly new should do the job.

The easiest way to do this is have wifi with a password on it so that others will be prompted for a password when they try to use your wifi. Most routers come with security options -- investigate the instructions for your router to accomplish this.

You can remove the cover to the outlet on the wall and remove the cord from the plug. If you want a more permanent solution, cut the cord completely.

Simply disable whatever interface is being used to interact with the one internet connection that you wish to get rid of. For example, if you are connected via Wi-Fi and by Ethernet, you can disable Wi-Fi to have only the Ethernet connection or unplug your Ethernet cable to only have the Wi-Fi connection.

Yes, there is a way to block internet connection to a specific device. First, I'd check if your brother is physically disabling your internet (ex. disabling your wifi on your phone). If not, then he is blocking you through your Gateway.

Put a password on your WiFi and only give it to select people, or you can set up a guest WiFi.