Then go onto live chat and ask them to remove ALL devices from your account, which at the same time ask for a password reset. To get live chat, you need to click "chat online" within the green box, don't hit "get help quicker" as this is a bot. https://help.nowtv.com/article/how-to-submit-a-complaint
FYI that I do not work for NOW, just a NOW customer trying to help
The other reason people think they want to remove a device is because they don’t have Boost, have finished watching on a device and started trying to watch on a second device, and been told they have hit their device limit*.
Removing a device would not anyway solve this.
If this is the problem, then the solution is to always exit what you are watching on the first device cleanly, which will send Now a trigger that you have stopped watching, and will allow you to pick up on the other device in seconds.
But if you exit what you were watching by just leaving the app, or switching off the device, then it can be up to 20 minutes before Now tries to poll what you were watching, and discovers that you have gone, only then freeing you up to start watching on a second device.
*or they do have Boost, and are still watching on, or have jammed up as above, all three devices. Just for grins, I have done this, and the locking up and freeing up mechanisms work exactly the same for a fourth device with Boost as they do for a second device without 😛
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.