How do you know there those devices are still logged in? Are you seeing them in the gateway's device list? You could always push factory reset (little red button on back of the gateway, push for about 15 secs,).
Do you mean logged into your wifi using its guest access as your thread title implies? If so access your gateway with your browser (192.168.1.254). Navigate to the wifi section and turn guest access off. I assumed you had to previously got into your gateway's wifi section to turn guest access on in the first place.
Or do you mean you had guests and you gave them your wifi ssid and password? If so change your wifi password and reboot the gateway to "kick" those wifi users off if they are still on. Reboot gateway by pulling plug for15 secs. and letting it fully reboot).
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