Warning
According to man xkill
Xkill is a utility for forcing the X server to close connections
to clients. This program is very dangerous, but is useful for
aborting programs that have displayed undesired windows on a user's
screen. If no resource identifier is given with -id, xkill will
display a special cursor as a prompt for the user to select a window
to be killed. If a pointer button is pressed over a non-root window,
the server will close its connection to the client that created the
window.
I've tested in Xubuntu 20.04.3 and in Setting / Keyboard / Application Shortcuts, there is a default shortcut to xkill. It's CtrlAltEscape.
A "X" sign will appear on your screen. If you don't want to kill any Window/Software, right click. If you want to kill a process, it's a left click on the window or software's window. If you're working on important stuff in your session, you should not use xkill or with precautions. I use it sometimes and it is fast and work well. Sometimes I like to live dangerously. Use at your own risk.
You can also open the Task Manager and Stop, Terminate or Kill a process.
Source for the following : Difference between stop, kill and terminate in Xfce task manager
From user Aloha on unix.stackexchange.
It sends different stop signals to a process. Here's some info:
Stop: SIGSTOP - This signal makes the operating system pause a process's execution. The process cannot ignore the signal.
Kill: SIGKILL - The SIGKILL signal forces the process to stop executing immediately. The program cannot ignore this signal.
This process does not get to clean-up either.
Terminate: SIGTERM - This signal requests a process to stop running. This signal can be ignored. The process is given time to
gracefully shutdown. When a program gracefully shuts down, that means
it is given time to save its progress and release resources. In other
words, it is not forced to stop. SIGINT is very similar to SIGTERM.
