Clarification of options

So we sell a supplement product in groups of 1 bottle, 3 bottles at a 10% discount, and 6 bottles at a 25% discount.

I believe someone is buying our 6 pack and breaking it up and selling it as a single to compete on our listing.

We do have brand registry, but from what I am finding I can't really stop this. Is that correct?

We are FBM, so right now with our 6 pack, we just send them 6 of our single bottles (with the single bottle bar code). It seems like the only option might be to create separate bottle labels for the 6 pack that says "Not authorized for resale" or something.

Would that work, or are there any other options I could do?

Thank you!