Sand to remove, or blast, or burn it off.
If you take them to a powder shop, they can take care of it with the least damage to the substrate. If its on there GOOD, then your really don't need to take it to bare metal, just a serious scratch and you can powder coat over powder coat.
If you got the capability, blast the hard spots (such as the bolt holes) then sand the flat spots with a DA or random.
If it was steel I would say burn it off with the careful use of a torch. But I would be more wary with aluminum, although a MAP gas hand-torch should do it pretty good job, then a light sand-blast or some scrubbing time.