Hmmm. I have not heard of any regulation that would stop you from doing so. I mean it's your business, you can choose to not accept new patients if you wish. You can also choose not to schedule new appointments for patients with "inferior" payers if you think that'll result in higher overall reimbursement from the appointments that you do keep. Plenty of providers don't accept patients with particular insurance coverage.
However, as an existing patient I would be very upset if my doctor told me that the wait for an appointment would be longer because of my insurance. I would seek another doctor, personally. Better figure a way to describe the scheduling difficulty as something other than an insurance-based decision, or you're likely to lose patients. As much as I'd like to think that providers treat all patients the same regardless of their insurance, I have experienced first hand a provider trying to upsell testing services that were not necessary, simply because he knew my insurance would cover it.
In short, I think it's unethical, but not illegal. If you've got more business than you can handle and your provider is still trying to find ways to make more profit, he isn't worried about patient outcomes, he's worried about the mortgage on his second home. That may come off a bit judgmental, but honestly, that's a greedy bugger.