anyone know a way to kill an In-Progress map/reduce script?

Technically I don't think it's possible. If you're daring, used to be you could kill a running scheduled script by temporarily changing the id of a custom field referenced in the script - change the field definition in NS, not the reference in the script file. So long as you don't have the whole thing wrapped in a try/catch block the script would error when hitting the reference to the field that you changed in the UI and kill the execution. I haven't tried that "hack" (and it is really a hack!) on a Map/Reduce yet, so I don't know that it'd work. But if it's serious enough, might be worth a try...

You can undeploy the script or mark it inactive

Such a better option!