You can also try to change the microphone reference level to LOW in the same menu where you found built-in zoom mic. Unfortunately there is no OFF option here so a very minimal of sound can be still recorded.
It is not possible to disable the built-in microphone of the Camcorder as mentioned by Charlie. OFF option of BLT-IN ZOOM MIC only restricts the microphone from recording sound following zooming in or out. However, you can try removing sound after transferring video using any video editing software.