Prevent walls to vibrate perpendicular to their own plane?

I'm wondering how to prevent shear walls to vibrate perpendicular to their own plane. I'm using the Shell Stiffness Modifiers f11=f22= f12=0.5 (as my code guide me). I'm not sure for m11, m22, m12. I've read topic thread802-141414 where WillisV said that he assigned m11=m22= m12=0.01. But there will be a problem in response spectrum analysis where (in such assignments) the walls will start to vibrate perpendicular to their own plane. Even if I assign m11=m22= m12=0.5 (according to my code) walls will vibrate in undesirable modes. What is the common practice respectively how to achieve that walls will not vibrate perpendicular to their own planes?