Is congress banned from regulating interstate commerce?

You answered your on question. What goes on within a single state can not be regulated by congress, when the enterprise goes over state lines it can be regulated. For example if a dairy farm in your state sells poisonous milk, and all his business is within your state, and your state has no laws concerning poisonous milk then he can sell all the bad milk he wants. If he tries to sell his bad milk in my state he has crossed state lines and I am protected by the food and drug administration. So I am protected against farmers selling bad food. Why don't you want to be protected???

Exactly the opposite: Congress IS delegated the power to regulate interstate commerce. Perhaps what you mean is that Congress should be banned from regulating intra-state commerce; a transaction wholly within a state.