Can you not ask the FDA to look into this?
They should (hopefully?) be able to force businesses to trace the product through the supply chain back to the actual physical source.
US regs isn't my area of expertise, but I do recall seeing various references to "misbranding" within the Code of Federal Regs, and indeed I'd have thought the following would be applicable, from 21USC343:
A food shall be deemed to be misbranded... (e) If in package form unless it bears a label containing (1) the name and place of business of the manufacturer, packer or distributor....
If they are copying your labels verbatim then they presumably are noncompliant with this requirement, as your business is not the manufacturer/packer/distributor?
There may be other legal options open to you as aside from food regs, it is presumably a fairly clear-cut case of fraud? Could be worth having a chat with a lawyer, if you haven't yet done so.