I don't think I'd call it a flat spin, but I got into a wierd nose-down spin in a Pony a few weeks ago (I have minimal time in Ponies) that no amount of rudder, elevator, or aileron could correct (yes, I do know how to get out of spins)... after about 10k of altitude loss, I finally just shut off the engine, and that worked.
So... Yes, I think spins of some sort are possible in AH.