How do I turn motion on one camera only off?

[TomMac]

If you only have the default modes, you have ARMED ( all on ) and DISARMED ( all off)

You can create/add a NEW MODE and use any or all of hte cameras to respond.

Any camera NOT IN THE CURRENT MODE WILL BE OFF

so if you have 2 cameras, create/add a new mode for camera A ( cam B will be off ) using time and recording settings that fit your needs.