In X11, there is no single unambiguous "process title". The closest equivalent might be the
WM_NAME property ("window name"). However there might be more than one window with the same name.
If using X11, you might try:
xprop -name "This is a window title" _NET_WM_PID
to try and get the PID of the process by window title. But depending on the process itself and/or the window manager you're using, it might or might not work: the text in the window's title bar might or might not have nothing at all to do with the WM_NAME property at the X11 protocol level. (The window manager decides if there even will be a title bar in the first place!)
And it looks like
xprop will just return the first one it finds, even if there are several matching windows.
If it seems you're getting the correct PID, you can kill it, for example:
kill $(xprop -name "This is a window title" _NET_WM_PID)
And if you use Wayland, these X11 tools and tricks will most likely not be applicable at all. (If you know more about Wayland, feel free to write your own answer or edit this one!)