We have already covered a detailed guide on kill, pkill and killall commands.

For those who are running X server there is another tool called xkill which can kill a process from its X window without passing process name or its PID.

xkill utility forces X server to close communications to its client which results into killing of client by its X resource. xkill which is a part of X11 utilities is very handy in killing unnecessary windows.

Full article here:
How to Kill Linux Processes/Unresponsive Applications Using ‘xkill’ Command (Tecmint)