I tried to solve this same problem before and ran into a few roadblocks. The issue is that the os copies the content's format along with the text to your clipboard, so if you try pasting html back into ANY textarea (or html-formattable area) youll get your content back formatted.
Luckily Jquery gives you some control over the document's clipboard, so ideally the trick would be to intercept paste, paste the content into a hidden textfield (which wont grab formatting) and then copy the contents back into the jhtmlArea. I tried doing this a couple weeks ago with little success, but Im gonna take another look at it now and see if I can come up with something.
Catch paste input Might point you in the right direction
Good luck