How do you kick a benign user off your system?

I was googling this a bit ago and noticed a couple of ways, but I’m guessing that google doesn’t know all. So how do you kick users off your Linux box? also how do you go about seeing they are logged in in the first place? and related… does your method work if the user is logged into an X11 DE (not a requirement I’m just curious)?

How to grant non-root user access to device files

I have device file that appears in /dev when a specific board is plugged in. The read and write operations to it work just fine, but in order to open the device file the program needs to be executed with root priveledges. Is there any way I can all a non-root user to open this one specific device file without having to use sudo?