Saying something flatly means conveying a direct, clear idea in a firm way. It's fine to flatly instruct your dog walker to stop dressing your bulldog in a tutu.
Someone who's comfortable being straightforward will have no problem flatly refusing to pay for a terrible meal in a restaurant or flatly directing a taxi driver to turn down the radio. Some synonyms for flatly include unequivocally and categorically. Sometimes this adverb is also used to mean "without emotion," as when you receive some stunningly bad news and flatly reply, "Oh, I see."