How do I disable negative scaling? By default blender allows you to scale past 0 - how do I prevent this?

i.e. it's common in other 3D modelling software to resize a set of vertices against an axis to align them - however this isn't possible when negative scaling is enabled.
As far as I know this isn't possible, use the keyboard input instead.

After pressing S for scaling use the keyboard to enter 0 (zero) to perfectly align geometry and optionaly choose an axis of alignment by pressing either X Y or Z.