I may have missed it, but isn't it possible they have their software set up to reject all of a customers wagers placed in the last 5 minutes before tipoff? Or just reject any of his NBA plays? In other words, I doubt this was some person sitting there intercepting bets live and rejecting/deleting them within a split second of the submittal - it is much more plausable that they had red flagged the account for some automatic reject/delete process based on some criteria.
As to the principle, I don't really see a case here as they didn't confirm so there was no agreement on a bet. I don't think they should treat customers that way, but I don't believe you have a case, IMO. They seem to be not-so-subtlely telling you to piss off, it would appear. They should tell you you aren't welcome anymore or that you can't make bets close to post or whatever they are restricting you from but unless they confirm there was no bet, IMO.
Lowering stated limits or not honoring posted lines is false advertising and sleazy, IMO, for that books should be exposed!