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
On the Bottom Toolbar, click the
In the entry of the popover window, input
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.):
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.