The phrase "a flat reject" is correct and usable in written English.
It can be used to describe a complete and unequivocal refusal or denial of something, such as a proposal or request.
Example: "After reviewing the application, the committee issued a flat reject, leaving no room for reconsideration."
Alternatives: "an outright rejection" or "a total denial".
Boko Harem flat rejected it.
"It is the policy of 99percentt of our customers in New York to flat out reject anybody with a landlord-tenant record, no matter what the reason is and no matter what the outcome is, because if their dispute has escalated to going to court, an owner will view them as a pain," said Jake Harrington, a founder of On-Site.com, based in Los Altos, Calif.
There will be times when reporters flat out reject your idea, or they don't quote a source that you've dutifully set them up with for their final story.
Someone who'd help me work through my feelings, not flat out reject them.
Or maybe he really did flat out reject you, as in doesn't want to see you again.
O'Reilly didn't flat out reject the idea but simply pointed out that the costs would probably be passed on to oil consumers (true, and fair).
These Supreme Court decisions flat out reject the arguments being made by Senator McConnell -- that campaign finance disclosure stifles speech, threatens the First Amendment and is unconstitutional because of the potential for harassment of donors and groups.
Google has rejected a flat fee, instead offering a percentage of Web advertising revenue.
The broad-brush idea here is that you don't always have time to answer a call – but you don't necessarily want to flat-out reject the call since doing that is not necessarily a nuanced enough response for today's hyper sophisticated mobile communicators.
"In just six short months, I will be officially a lame duck, which means Congress now will flat-out reject my authority.
