Press escape. Click quit. Click yes.
Or
Go to the Main Menu. Click Quit.
I think he meant uninstalling it.
If you did mean that, if you have windows, go to the start menu > type and click "uninstall a program" > find blockland > uninstall > go to blockland's site or find the installer > install blockland
i am not sure if this will delete your addons, so back them all up and put them back if they are deleted.