It raises an interesting question about whether "unpublished" restrictions can be said to form part of the contract of travel, and whether, in the case of ambiguity or conflicting information, as is apparently the case here, the consumer is entitled under the Consumer Rights Act to the benefit of the most favourable interpretation.
This is especially so given that NRE purports to be the "definitive source of information".
http://www.nationalrail.co.uk/46383.aspx
If someone asked for such a ticket at the ticket office (or TVM), and the ticket office/machine advised according to the conditions as stated in the NRE text, then the customer could legitimately travel at the time that is intended to be barred, that's without doubt in my opinion.
Also if a customer bought online there may similarly be a case, depending on the circumstances e.g. if they used a website which allowed a ticket to be sold without an itinerary (e.g it may be the return portion of an "open" return) or if they later decided to travel earlier.
Virgin wouldn't be allowed to do anything other than either accept the ticket, or issue a ticket irregularity report (which could includin taking the ticket from the customer, but if they do this, they must honour travel and issue replacement tickets/travel documentation where applicable/appropriate).
However, in practice, it could be a nasty experience for the customer, who might be asked to pay an excess up to the Anytime fare (or a new Anytime Single, if cheaper), and if the customer insisted on their contractual rights , things could escalate (I've known Virgin call - or threaten to call - the police on people who were travelling according to their contractual rights on numerous occasions).
I don't think I'd recommend that experience to anyone, at least not without some training in conflict management techniques.
That said, if a customer became aware of the restriction and the fact the NRE text has an omission, it might not be a good position for the customer to be in; you might not want to admit or be found to be taking advantage of an error. That's not to say you'd necessarily be in the wrong for doing so, but it's not something I would advise to an average customer as things can get pretty nasty, especially with Virgin Trains.
