You don't specify what machine architecture, so I'm going to assume IBM PC/AT compatibles.
In the world of the PC/AT compatible, the on-board audio is (nowadays) physically incorporated into the silicon chips that comprise the so-called "chipset". An Intel ICH9, for example, has an Intel HD audio device as PCI device #27, function #0. It's not physically possible to have an ICH9 without on-board audio. The most that a motherboard manufacturer might do is not wire the signal pins from the chip to I/O codec circuitry. Similarly, "disabling" the device in the firmware setup utility means very little in most situations, as modern protected mode operating systems directly enumerate the PCI bus, will find the audio device, and will attempt to drive it.
On-board video is a different matter. Such functionality is not universally integrated onto motherboards, and is rather the function of a display adapter device, usually an expansion card that is plugged into an expansion slot on the motherboard. So there's nothing to ask here in terms of the motherboard itself. Simply buy a motherboard without buying a display adapter expansion card to go with it. ☺