I agree that it is not always easy to disconnect devices, but I wish it were. In my case it is not that someone unknown has connected, it is my own devices. I have 3 android devices that show up twice because at one time they were connected to my main router, now I have them connected to my range extender. Same with 2 IP cameras, except there are 3 instances of each, 1 to each wifi device and 1 when they were connected via ethernet cable for initial configuration. Plus some friend's devices when I was working on their computer and laptop.
But if I recall, if a device hasn't been connected in a long time, they seem to eventually drop off the list.