there are currently several parts of WHMCS that do not work, unless you setup USD currency with an exchange rate.
such as the marketplace products, ENOM domain sync tool..
This then allows customers to pay in USD.
For those of us who are based in the UK, we don't want our products available in USD or customers to pay in USD.
As you have already confirmed, there is no way to add USD currency, but disable it from being used.
Please fix this and either allow the required currency to be disabled so customers/products cannot use it, or better still, just calculated your USD exchange rates internally in your code.