The good thing here, is that these hackers aren't actually after your money, rather, they're just freeloading with your user details.
So, if you want to know if someone is using your details, have a look at your 'recently watched' section. If looks like a load of things you've never watched, and would never, ever want to watch, chances are, someone's jacked your account.
To stop this from happening, it is a very simple process - go to 'Your Account' and hit 'Sign out of all devices', which will log you out of every device that has been using it. So if someone miles away has been watching Orange Is The New Black on their phone, you'll log them out.
Obviously, from there, you'll have to change your password. Try to ensure it is a different one from your email password and what have you, to make certain that hackers haven't got it.
Then, you're away! You'll be the only one watching what you've paid for, and no-one will ever know that you've been secretly watching Robinson Crusoe On Mars over and over, like some kind of mad person.