Depends what options are available on the router. See link below for getting to the router settings.
Open a new web browser window.
In the address bar, type 192.168.0.1 and press Enter. This will bring up the settings screen for your Hub.
Select the Wireless tab. Enter the username admin and password nowtv (in lowercase) when prompted. If these login details don’t work, you’ll need to reset your Hub.
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.