It sounds like you've already done as much as you're likely to be able to do by alerting your insurance company. they're on the lookout for all forms of fraud these days, and this sounds like it may be some sort of fraudulent activity. One things for sure. Find a new dentist you can trust. Good luck.
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Your insurance company has protected you. Hopefully your dentist is now locked out of your account. Find a new dentist and consider reporting this conduct to the dentist's licensing board. I am not sure who that would be, but you can probably go online and locate this information. Best wishes to you.