Cropping an image does not remove part of the image, it merely hides it. Perhaps this will suit:
Take a screenshot of the message and paste it into a Word document


Crop it
Take a screenshot of the Word document and paste into a new Word document.

Does that do what you need? (If you crop it, it will still "uncrop", but at least all they'll see is the rest of the blank Word document, not the rest of the message.)