Can a provider who accepts Medicare assignment refuse to see an established patient for not paying their coinsurance before the visit?

I was diagnosed with cancer and had to see a cancer surgeon, when I was asked to pay my coinsurance, I responded to the registrant that I thought that came after the claim was adjudicated. You send me a statement, Medicare sends an EOB, I match them and make payment. She said no, and called someone. This big, burly came down to registration and he intimidated me to pay and told me I had to pay or be escorted out. I paid. I had a follow up the next week and was told that I had to pay, this time I had no extra funds, but I told them I would pay when the bill came. They asked me to leave. Is this allowed?