In this case I have 18 little brake disc bolts to clean and polish and need some way to hold them. This is called splitting a nut (I see a joke there...) and it comes in handy for lots of little jobs like this.
Begin by finding an old nut that fits the threads in question, then simply cut a slit in one of the flats using a Dremel or hack saw.
When squeezed gently into a vice, the bolt is held tight, allowing me to quickly and easily complete this task.