How does a flying wing keep from going into a flat spin when maneuvering? How do flying wings, like the B-2 Stealth bomber, actually keep themselves from yawing out of control without a vertical stabilizer?

For the record, I assume this has to be a simple mechanics process.  Why?  Well flying wings go all the way back to the 30s.  One of the earliest (and my personal favorite) is the N-9M, which was a scale model of the XB-35, a prototype bomber for the allies during WWII.

They didn't have flight control computers back then, and the only control complaints I recall them having is that early versions had a tendency to flip over backwards when approaching stall speeds, well, that and the ground effects were pretty strong. But, no mentions of going into flat spins when going into hard maneuvers (that I recall).

So how do they control that Y axis on flying wings?  Are they perhaps more susceptible to a flat spin than a regular design (even if those risks can be kept to an acceptable minimum)?
Sweepback helps a lot, and the rest is achieved with drag devices near the wingtips.

(Source: aerospaceweb.org)
Did you notice that the B-2 has split ailerons, and they are partially opened during flight? Their drag pulls the wingtip back, and in combination with sweep the forward wing in yaw will have a longer lever arm, so the same drag produces a bigger stabilizing yawing moment.
Differential thrust is not available on gliders, and there are lots of flying wing gliders. Horten III and IV, Fauvel AV-36 and the SB-13 are some examples. The Hortens used speed brakes near the wingtips, and the SB-13 has rudders with differential deflection (70° outwards, 20° inwards) on its winglets to increase the yawing moment.
Directional control is no problem with flying wings, even though directional stability becomes weak at high speed when induced drag is low. Flutter and a low $c_{L max}$ are the real drawbacks. Ground handling, landing characteristics and payload space are others, but can be overcome somewhat.
And flat spins are not possible with flying wings. Their spin modes are all fairly steep due to the lack of a strong inertial moment from the lengthwise distribution of masses.