I don't get it. The stop cam (washer) cannot rotate because the pin is engaged in the hole in your fork. If the fork plate itself is moving it's a defect, since it's supposed to be bonded to the fork or center post.
If you've somehow sheared the pin off the stop cam (part of the brake) you need a new one, and your best bet to to email the maker (or importer) and explain and buy a replacement directly. In many cases manufacturers will send the part at no charge.
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