I suggest you download the app Fing. Preferably on Android, but available on iOS at a pinch, though more restricted there by Apple’s privacy rules.
This will tell you every device on your broadband, and if you spot an intruder, you can delete them in the app.
Fing is free for basic use, but if it’s worth it to you, you can subscribe, and then it can be set to alert you to new devices connecting to your router, rather than you having to monitor for them yourself.
NB: I take it you are asking about people piggybacking on your WiFi here? There’s a similar annoyance of people piggybacking on your Now subscriptions, if you have those, but the fix is different.
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.