William,
Check page 287 of the User's Manual. This is under Custom Settings a Autofocus, sub-menu a5 AF Activation.
What you can do is set the camera to focus only using the AF-On button. In this case, pressing and releasing the AF-On button will lock focus and then pressing the shutter button will not refocus. Of course, if you continue to hold the AF-On button down, then the camera will refocus at whatever it is pointed at. If you choose to allow AF Activation by both the shutter button and the AF-On button, then the pressing the shutter will refocus after you have released the AF-On button.
Using the AF-On button as the only AF-Activation option gives the D700 what is called "Back button" focusing. It allows you to get a focus and then recompose the photo without disturbing the focus. Useful in landscape, etc. cases where the scene is static. Very hard to use, for me, trying to photograph a flitting butterfly or other erratically moving subject.

Ron