I need to know how to log someone who isn’t near me off my xFi app?
If they know your SSID and its password, about the only thing you can do is change the password. That means you will have to also change the password in all devices that have the password. For instance, my TV uses wifi and I would have to change its password.
@Sheli2, the xFi app and website are only available to the primary account holder and any unrestricted secondar user on the account that has a valid Comcast user ID. If you need to prevent one of those users from accessing xFi and changing controls on your home WiFi network, you would have to go into MyAccount and make that user a restricted secondary user (or remove their account) - this would prevent them from using xFi, along with the other controls available to unrestricted secondary users.
If you are looking to block users and their device(s) on your WiFi network, as RoberyWy mentioned you could change your WiFi password so that they no longer have access to your home network. However you would need to reconnect everything. You could also pause their device so that their internet access to your network is blocked whenever they are within reach of your WiFi signal.