There would never literally be no reason, though. If a human being is looking at your account and banning it, they are doing so for *some* reason. Realistically, if that reason was something stupid like "I was having a bad day and just felt like banning a random person" then you would be far more likely to see results by contacting SE and having that person reprimanded internally. Actually thinking you could successfully pursue a legal action against the entire company for something like this is silly, and you would never get it off the ground.

And not sure why you think they're taking your money when they ban you. They gave you the service up to the point you got banned. It's not like a banned person somehow retroactively has their game time taken from them when they're banned. Once they're banned, SE does not continue to charge you afterwards. Your account and subscription are canceled from that point on and no money is being taken out for a game you can no longer play. Really, you're just using extreme language like "stealing" and "theft" to make it sound like something that is incredibly mundane and commonplace is some heinous crime against humanity. It's not. It's a video game subscription and no court is going to care that you can't play your video game anymore.