Hack the Firmware and Deactivate the SDM

http://www.pentaxforums.com/forums/maintenance-repair-articles/208353-how-deactivate-sdm-autofocus-allow-screw-drive-autofocus-da-16-50mm-f-2-8-a.html

I think the above is of high risk. Do only when you clearly know what you are doing and try it at your OWN RISK!