Nope. If you purchase a rank you promise to follow the rules. It states in the EULA that they don't have to give you your money back.
Nope, you can't demand, it clearly stated that cube doesn't have to refund. You just shouldn't have gotten banned, your fault.
You accepted a agreement when you downloaded minecraft. It stated that the sever does not have to pay you back. This is called The Minecraft EULA.