Windows, disable rich-text copy
Take a look at PureText. I don't think there is a specific "OS-level setting" for disabling rich text in the clipboard, but the goal of PureText is to do exactly that.
It also gives you the flexibility of setting custom hotkeys for converting text to plain text format. By changing the hotkey to Ctrl-v, you can paste and convert the text in one shot.
Compatible with Windows 95 and up. PureText is freeware.
Press Ctrl+Alt+V
in Word to Paste Special, i.e. paste without formatting.