According to this answer on the Microsoft forums, there is such no option for Word 2013.
When you try to save the Word document in plain text you will receive the warning about losing the formatting, there is no option in Word to disable the prompt/warning.
- Deepa Raj
Looks like just another case of inconvenient design on the part of Microsoft to encourage use of their preferred file format. Hopefully they can address this in the future.
I suspect there is a way to intercept the function call for the message box and corrupt the memory block that holds those instructions, though I'm not sure what affect that would have on other functionality.