How can I tell iTunes to stop playing a song?
The AppleScript for this is pretty simple:

tell application "iTunes" to stop


You can make this easily accessible by creating a Service.

Open Automator and create a new document. Select the Service document type. Set the Service receives: menu to no input. Drag in a Run AppleScript module and paste that code into its text box. Save the service and give it a name.

You'll now be able to activate it by going to the current app's menu and choosing Services-> Stop iTunes (or whatever you named the service).

You can assign a keyboard shortcut by going to System Preferences-> Keyboard-> Services, finding the service (it should be under General at the bottom) and adding a shortcut.