I don't know whether the shortcut can be disabled, as it is "baked into" the user interface. "S" is the built-in accelerator for the "Send" button.


Try this: use third-party macro software, such as AutoHotkey, to create a window-specific, or perhaps a global, script triggered by Alt + S, to insert ś.


I find AutoHotkey usually intercepts keystrokes before applications. But there's no guarantee that Outlook won't occasionally "see" it first.