How does Egypt prevent people from arbitraging their currency? Apparently Egypt uses a fixed exchange rate so there is a substantial difference between the government rate for the Egyptian pound and the price on the street. What prevents someone from just buying pounds on the street using dollars and then depositing the pounds in a bank and converting them back to dollars at the official rate, thus allowing the person to instantly make a large profit?

I assume that there is some mechanism to prevent this. Is there?
Used to be capital controls. You could only buy dollars (or any other foreign currency) through official channels at the official favorable rates by providing documentation displaying need for said currency (e.g. Travel or import documents).