After forced quit, “killall Finder” says “No matching processes…” but PID still exists?

There's easy and slightly harder. First easy, you could probably use Activity Monitor to force quit it.
Harder, and the reason you're getting this message, is because your user didn't launch Finder, the system 'owns' it - you can only kill it if you elevate your privileges to super user(!), sudo killall Finder, and then it'll ask for your password(assuming your user account is an admin, otherwise this wouldn't work) just like Activity Monitor would. In certain *nix distros you can kill straight from top, as well.

Worse comes to worse, you could force log yourself off in a pinch, you could killall loginwindow(your user owns that one while you're logged in)