People I don't know ow using wifi

Perhaps your neighbors are inadvertently connecting to your network / gateway device via the MoCA feature that's built into the gateway device. MoCA (Multimedia Over Coax Alliance) is an alternate hardwired way to connect devices to a home network if ethernet cabling can not be used/run. It uses the existing coax cable wiring instead of ethernet cabling. 

If the MoCA feature is enabled, and you do not have a PoE (Point of Entry) MoCA filter installed on the coax line, yours, and your neighbor's gateway devices will be able to connect with each other like one big network. 

Disable it if you aren't using it. If you are, install a PoE MoCA filter on the input port of the first splitter off of the street drop to keep the MoCA signal in your premises and to keep the neighbor's signals out. Your neighbors should also have POE filters installed.

To disable MoCA on your device. First, unplug the coax cable line from the jack on the gateway device. Then go to the gateway login http://10.0.0.1    

Default login info is:
Username: admin (all lowercase)
Password: password

Once in you will find the drop-down on the left of the screen that is labeled “Connections”.

Click the drop down---> click MoCA---> right side of screen click “disable”---> save changes.

Bear in mind that we've seen many posts here that the MoCA feature gets turned back on with their overnight re-boots / updates for the device. The best policy is to be sure to use that PoE filter ! Amazon has them: https://www.amazon.com/SNLP-1GCW-Filter-Eliminate-Multi-Room-Interference/dp/B07SLD9QPH     

Or you may be able to get one for free at your local Comcast / Xfinity store. Or book a tech to come out and install one for you.

If the MoCA feature is not being used on your home network, you can put the PoE filter right at the back of your gateway device instead.

@OnlyNightblade  The best course of action, in this case where unknown people are using your services, would actually to also report this to our Customer Security Assurance. You can contact them by following this link: https://internetsecurity.xfinity.com/help/report-abuse/ or by calling CSA Toll-Free Number: 1-888-565-4329.