You should contact a lawyer in your area that deals with real estate contracts to advise you, as agents cannot give legal advice. Generally, once a contract has closed you would then have to resolve any disputes or disclosure issues through litigation.
By the way, did you use a buyer's agent to assist you with your purchase? If so, did you ask your agent to verify the flood zone status with the sellers, FEMA maps or local governmental agencies? If the flood zone issue was important to you (and it sounds like it was), then the time to verify the flood zone status was while you were under contract and before closing.
Knowing the flood zone status of a property, especially the Base Flood Elevation of that property, is more important now than it has been in the past due to recent changes to the national flood insurance program. That is especially true for older homes that had previously been "grandfathered" into artificially lower rates.