How does a flying wing keep from going into a flat spin when maneuvering?
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.