"I'm not comfortable filling this prescription. I am concerned because... (insert reason you don't want to fill the script.)"
I've had a pharmacist refuse to fill a prescription* for me and that was how she went about it. I thanked her for her trouble, took my prescription back and found a different pharmacy... which is pretty much what will happen if you refuse to fill. Some people might get more impolite about it than others, but basically, you have a right and a responsibility not to fill a prescription if you have concerns about it that you can't resolve with the patient or the prescriber. You might lose some customers, but if you truly believe that the alternative is to provide excessive amounts of controlled substances, then losing those customers might be a good thing.
*Testosterone, a schedule III drug, not a narcotic. I've been on a steady low dose for many years, and only had trouble getting it filled a couple of times.