we still don't know what car you are talking about : W ???.
On my W 124 193 ( a 300 TD turbo) as on most MB's of that
period, the center is allways blowing cold. While warming your car you can keep your head fresand cool!. I like this system.
To shut off you have only to turn the small wheel in the center.
If that don't stop the airflow than check the wire connected to this wheel or lever. On the right side of the wheel you will find a small Allen-screw that should hold this cable in place/connected to the wheel. Is this screw fastened than you should check the cable...
Please let us know your findings.