Bad news on several different areas:
1. The stingray is not a suitable home aquarium fish. Their care is beyond hobbyists, not to mention the fact that they require open ocean for proper thriving.
2. As a potential lethal fish, their import is restricted in some areas, as are their collection.
3. Their underside is so sensitive that the common aquarium substrate will harm them -- even some sandy substrates.
4. The removal of the parasites will likely lead to internal bacterial infection, which complicates the matter.
5. The removal of the parasites is best done in a operating environment, the subject under anestesia, by a professional (vet).
6. The use of uncomplexed iodine on an open wound increases the likelihood of systemic poisoning -- we use betadine for this reason, which contains the povidone-iodine complex.
7. The extent of the described infestation is likely going to lead to the death of the ray, whether you can get them all off or not.
I'd return the marine life to where you got it, get your money back, and stick to making more sensible, 'usual' ornamental marine fish selections suitable for captive life.