Zorin OS should come with xkill already installed. However, if you need to install it for whatever reason:
sudo apt install xorg-xkill
Now that you have it, you can ensure you have fast access to it for those times of need.
Zorin OS Lite:
Open the App Menu and navigate to Settings > Keyboard
On the Keyboard settings manager window that opens, select the tab Application Shortcuts
.
On the Bottom Toolbar, click the Add
button.
In the entry of the popover window, input xkill
Click the "OK" button.
Now a new Pop Over window will appear that asks you to press the Shortcut you want to use. I prefer ctrl+alt+k
Once done, it is all set up.
If you do not get the popover window for the keyboard shortcut- click the 'xkill' entry listed in the shortcuts and click edit
on the bottom toolbar.
In Zorin OS Core (Or other Gnome D.E.):
https://help.ubuntu.com/stable/ubuntu-help/keyboard-shortcuts-set.html.en
It's that easy. Now, if a misbehaving app needs the axe, hit ctrl_alt_k on the keyboard, then Click the Window of the process you need killed. It's gone.