This can be found in the Applications folder.
In the devices list, select the iPhone/iPad you're working with.
This button can be found in the bottom left corner and is denoted by a small black triangle inside of a box.
In the device settings popup menu, select the "No Application" option. Close Image Capture and save your settings.
Sometimes, when you are busy or out and about and your iPhone starts ringing, you don't want to pick it up, but you know it might be so-and-so with that important call you've been waiting on.