I'm sorry to hear you were given bad information by a sales associate.
I personally try to avoid any kind of high pressure sales situation. I prefer to make a thoughtful, informed decision on my own time rather than rely on whatever some salesperson is willing to say in order to close a deal.
I mean, look at how misinformed some Delta reps are even though they don't have much of an incentive to insist on some of the rules they do (e.g., SDC rules, GUC rules, etc.).
Have you tried lodging a complaint with AmEx? I've found they have good customer service, and at a minimum, they should be able to refund your annual fee and cancel your account.