Is there a way to kill an unresponsive applescript launched by an application without killing the application?
It looks like the AppleScript is run within Quicksilver’s process (depending on how you run it). If that’s the case, you’d need to relaunch Quicksilver. If it’s not completely frozen, you can do this quickly with ⌃⌘Q.

It might also be possible to set some kind of timeout in the script to prevent it from hanging indefinitely.