There are those who believe that none is singular and should always take a singular verb.
This is the usage note on the subject taken from oxforddictionaries.com. "It is sometimes held that none can only take a singular verb, never a plural verb: none of them is coming tonight rather than none of them are coming tonight. There is little justification, historical or grammatical, for this view. None is descended from Old English nān meaning ‘not one’ and has been used for around a thousand years with both a singular and a plural verb, depending on the context and the emphasis needed."
To my BrE ear it sounds pretentious and bordering on the hypercorrect to insist on the singular at all times.
In your example I would naturally say, "None of the codes have been denied".
In many cases, the use of "none of xxx" can be singular or plural. When the noun following the preposition is an uncountable noun, the singular form is used. There are, however, cases in which a plural verb is needed.