Unable to stop air flow in back vents

[A||||1||Robert W. Jones||Diesel Timing Light?||||5||||Stray Cat||00012887]

Is it by design that you can't stop the air flow from the back vents? (The one under the passenger seat, drivers seat and on the console.)

I tried different settings on the A/C controls but it always seem to have air blowing out of the vents back there. This was really noticeable on a trip down the 401 at highway speeds. A friend was getting cold in the back, and it was warm in the front.

Any ideas?

Thanks
Sheldon