How do I "lock" focus on first half-depressing the shutter button?

On my K-5, often I want to fix focus by half-pressing the shutter button, and then I want to shift the framing of the shot.

If I try that currently, the camera continues to try and auto-focus. I'd prefer if focus "locked in" on the half-press. Does that make sense and is it possible?

Set the switch next to the lens mount (on the left side) on AF-S(autofocus single). It presently is set to AF-C(autofocus continuous).

Thanks Yves