If it just a Rivnut with a bolt in it then I would think you could just cut the bolt off flush with the Rivnut and then grind away the flange of the Rivnut.
Life is rarely that simple though. Odds are the bolt goes through something else, like a body armor panel, that you want to keep and that is in the way of my seemingly simple solution.
That then begs the question of access to the back side. Is this in an area that would be accessible if you removed an interior panel?
If there is no access to the back, and this does indeed go through a panel or light mount or some such thing, then what I would be inclined to do is to wedge the panel (or whatever) tight against the head of the bolt and then try to drill out the bolt (purpose of wedging is to keep the bolt from spinning). Drill the bolt out far enough to be able to break off the head, then pull off the panel, then go back to grinding the flange off the Rivnut.
