Well I have recently fitted new handlebars and Barkbusters.
Are you keeping the standard bars?? (they bend if you fall off alot like I do)
Anyway, apparantly if you insert the bolt back into the bar end insert and tap it inwards a little, it should free up, and you can remove it. I tried this, and even after a 24 hr soaking in WD40, and lots of heat, they didn't budge.
Why don't you get some stronger bars, forget those inserts, and do what I did?
http://www.xt660.com/showthread.php?t=15842