I actually don't think you have a case.
You are still able to access Blockland on your computer; that is not restricted. Badspot banned you from the auth server, which is his private property, meaning he has every right to do so.
Not if you pay to use it. Go play any other game, World of Warcraft I know states that accounts are owned by Blizzard and can be terminated at any point in time, same with most other games. Blockland on the other hand promises only a "lifetime membership" which I have, despite legitimately paying for, had revoked. I definitely have a case, whether or not I'd launch a case against Eric Hartman is a completely different story.
Plus when you buy Blockland, you are paying for single player only, multiplayer being an added bonus, and since the key still works for single player...
The key does not work for single player. An authentication failure puts you in demo mode. Not to mention the other parts of your statement are completely false.
Here's a revoked authentication key ABB5Z-2XRM-ZYWT-L522