It's hard to tell what exactly it is, but I assume you've scaled the face as flat as possible with the scale manipulator or the flatten tool.
It's possible some of the ripples are caused by the patches being stretched relative to their neighbouring ones. To avoid this, you should also flatten the neighbouring patches round the area you want to flatten. You can however, use the bias handles from a side view of the offending areas and align them with your flattened patches.