Change your password. https://www.xfinity.com/support/articles/view-change-wifi-password
Years ago, I set up a "guest account", like I was encourage to do. Now, I can't delete it and my neighbors
live so close, all they had to do was hook up to the nearest hotspot which was 20 feet away, find my vulnerable 'guest'
account, and now they own me. Acess to EVERYTHING. My Roku, my cell phone, they even have squirmed up in here from my car. I change passwords, but they have control over everything.
We apologize for any inconvenience that you may be experiencing. I am not sure why you may have been instructed to set up a "guest" login unless you were wanting to provide a login to your friends/family. Now, all of our WiFi enabled modems have access for guests by default. The network is titled "xfinitywifi" and is available to all Xfinity customers with a valid login. This network is completely separate from your private "in-home" network, does not use your data, and would not affect your speeds if used.
If you are worried about a neighbor gaining access to your private network, we recommend what user: nerdburg suggested and either logging into Xfinity My Account (app or portal) and change the password for that guest account and/or delete it all together. Also, you can always manage the devices on your private network by using the xFi App.
Finally, if you truly believe there has been a breach of your account's security we highly recommend that you contact Comcast Security Assurance team (1-888-565-4329) so they can investigate, give you tips to prevent that in the future, and take any legal action that may be necessary.
I hope this helps. If you need anything else or have any other questions, please don't hesitate to post them.