I'm assuming you have the basic, one account for everyone setup on your computer.
If I was in your current situation i would do the following.
Step 1 - Backup all of your Blockland addons and user settings in another folder.
Step 2 - Uninstall Blockland
Step 3 - Set up another user account on your computer. Make sure it's passworded.
Step 4 - Install Blockland for the main account(not all users, JUST the main account)
Step 5 - Change the firewall settings so that the firewall totally blocks the main user account Blockland from connecting to the internet.
Step 6 - Install Blockland for your personal account.
Step 7 - Never let your brother know your personal account password.
Granted this would only work if your brother knew nothing about the firewall, and you only had one computer that you both use.
The point is for you to block any of his Blockland.exe's from connecting to the internet. While still allowing you to connect.
A less elaborate method would be to simply block the computer's Blockland exe from connecting to the internet, and disabling the firewall every time you want to play. Kinda... unsafe as far as viruses go but i guess it works.
Or you could just get your key back from him and delete the key.dat file. But there's no guarantee that he hasn't just written it down somewhere else.