One way I have found to block a new device is to fill all the device slots and leave no changes available.
A slight problem these days is you get three changes a month so you have to remember to swap the devices at the start of the month.
However nothing to explicitly stop a particular device.
Surprised changing password didn't stop it.
Go to My Account/Devices and remove any devices that you don’t recognise as yours. With your password changed, nobody should be able to put them back.
If they are all devices that you recognise, though, check the Last Used Date and Time for entries that don’t match your recollection of the last time you used that device or devices, which will show someone sneaking onto your device somehow (e.g. TV In a shared house with your Stick in the back?)
As your security seems to have been breached, I suggest you also set a Payment PIN under My Account/Settings and PINs, which will forestall any attempt by a third party to set up Passes on your account, and have you pay for them.
I only have 4 devices to put on it. I can have 6. Is there any way to change that?
@Char17 What are your four devices?
Is one a PC/laptop? If so, then you may be able to trick the account into thinking you have extra devices by using the Now TV app and also by using the account in different browsers (Chrome, IE, Edge, Firefox). However, I have never tried this, so I can't be certain it will work, but it is worth a try to ensure there are no empty slots that others can use.
If you set up six devices and somebody has access to your account, they can simply remove one, and then add their own.
If not this month, because you have used up your three changes, then next.
If you think your account is still being breached, even now, then change your password again.
My advice is above; if you can’t take it, or you think it will be unsatisfactory, please tell me how, and I will look again at it.
Char & Fellas
I find it a bit of a mystery when Char said, "I have changed my password but they keep using it...". The only way someone can keep using a changing password is if:
1) You write it down and it gets found.
2) You tell somebody, i.e. a secret shared is no secret.
3) Your password is easily guessable.
Personally, I have found that usually an eight or more digit password consisting of a combination of letters, numbers and non-alphanumerical characters, which is not shared, are usually unguessable.
Therefore, unless you are dealing with a "computer hacking genius" I don't get how this miscreant can constantly find out your passwords when changed.
I believe that when this "mystery" is understood then a solution can be applied.
changing your password on the router should do the job as long as the other person has no idea you have done it. You also need to pick the most random password you can think of a combination of letters / numbers / upper and lower case.