You can use Activity Monitor to kill Tunnelblick. It's in /Applications/Utilities. Launch it, click to select "Tunnelblick", then click on the "X", and click the "Quit" button. If that doesn't kill it, click the "X" and then the "Force Quit" button. You should then kill any processes named "openvpn" with the same sequence (try "Quit" first, then "Force Quit" only if necessary.
Usually, Tunnelblick isn't "hung", but instead has an open window asking you for something. So double-check that no such windows are open.