Use MOVE command as follows:
- start MOVE command
- select objects you want to flatten and hit Return
- pick any point in the drawing and hit Return
- from the keyboard type: 0,0,1e99 and hit Return
- call MOVE command again
- type P and hit Return twice when AutoCAD will ask about selecting objects
- hit Return again, when AutoCAD will ask about Base point
- - from the keyboard type: 0,0,-1e99 and hit Return

Actually you move objects you want to flatten 1e99 units high along Z axis and then move them back.

Maxim