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)?

Why is the ‘sudo’ password different than the ‘su root’ password

On my personal machine, I often type sudo in front of certain commands in order to accomplish administrative tasks. I had hoped to avoid doing this throughout the day, by typing su root and providing the same password I usually do for sudo. However, the two passwords are not the same(I don’t know how to log in to su root). Is running a command with sudo different than logging in with su root and running the same command?

Is it possible to rename a Unix user account?

I installed Ubuntu on a computer that is now used by somebody else. I renamed the account with her name, but it only changes the fullname, not the user name, which is still displayed in the top right (in the fast-user-switch-applet). Is there a command to rename an Unix user account?