I did this very mod last weekend.
I assume you are trying to remove the stock nut thats inside the bar, so you can mount your own adapter/mirror setup?
As others pointed out, you have to "Get Nasty" with these things. Get a drill out and drill out the black rubber ring in between the bar and the internal nut.
The rubber is actually about 1" long and goes all the way around the nut. Just start drilling holes into the rubber and then go back and fort with your drill to break sections free.
Once you've removed enough rubber, you can pull the internal 1" long nut out.
Clean up the internals of the Bar and you're Good to Go!