How can I remove a stripped nut from a bolt?
You can use a special tool called a nut-splitter.

Another method is to use a rotary tool (e.g. Dremel) with a cutting wheel or grinding bit. There's a strong likelihood of damaging the bolt.

As a last resort you can use a smaller angle-grinder to cut through the nut and/or bolt. There's a good chance of damaging the fork (or yourself) unless you are very carefull.