This brake was installed on a fixed-gear that did not come with any brakes. The nut on one side rubs against the fork, and over time it rotates and presses the pad against the wheel.
Here is where it is rubbing:
The workaround is to unscrew the break and re-center it, but it wiggles its way back and I'd rather a more permanent solution.
The first thing that occurs to me is to put a washer on the bolt between the brake and the fork, but the bolt is just barely long enough to screw into the nut. Could I get a washer and a longer nut to handle this? Would that fix the problem?
The other idea I had was to file down the nut in the photo so it doesn't contact the fork, but that just feels like a bad idea.
[Edit] As requested, here's a shot that shows the whole brake: