Doing some checks as I have a speed problem, and found my neighbour's PC showing on my network. I have recently changed to Plusnet and am using their router with standard wireless security. This PC shows in the network list even with WiFi off.
Next time you see it showing, right click and click . But also, as wiz-king says - change your network password, and if you are not sure how to do this, speak to PlusNet and tell them why you need it changed.