Change the wifi password. Problem solved. It can't connect.
You'll have to forget, then rejoin your devices. iPhone is a bit touchy that way. windows/linux/mac may just say the key is bad and ask you for another key, but they get hung up on the key exchange sometimes. It's better to forget the network, then rejoin.
Are you sure about not owning a Linux device? Being smallish (and free!) is quite popular for use in: routers, TVs, TV streamers, DVRs, home security devices, TVs, thermostats, cars, refrigerators... If you still think not, then you should probably change your password as @flatlander3 suggested, and then slowly add devices back to the network one at a time and see if the "linux device" re-appears.