Yes, it is the main router in my network.
That sounds promising, though it's not showing up in that list at the moment. Could that be because the device in question is off? Does that list only show current (recent) active users? I do, however, see that computer in the user list in the Almond app on my iPhone. When I click on the wrench icon, it displays a bunch of things, but it doesn't appear to let me Block it from there. On the web interface, it took me a bit to find, but apparently a similar UI is available under "Sensors" but still no ability to Block.
It would be important for me to be able to change this remotely. Is there some way to do it, maybe in the Advanced area of the web UI? I've managed a number of routers with variants of OpenWRT & similar, so not afraid to dig in...