In the right bar (Press N to show it) set the "Mean Crease" value to 1 (it sets how much the selected edges don't react to subdivision smoothness Edit: note that the face is still subdivided but the subfaces created by subdivision mantain the original position. No smoothness.). They'll become violet.
To solve this:
- I had a cube with subdivision done to it as shown below:
- Let's say I don't want the upper face to be subdivided. First switch into edit mode and choose face selection as shown below:
- Select the upper four faces which you want to be one face as shown below:
- Go to the mesh menu, then Faces, then
Make Edge/Faceas shown below:
- Then they will be back into one face again as shown below: