The present paper reports the preparation of sintering additive-free Si–B–C–N monoliths by spark plasma sintering (SPS) under nitrogen of polymer-derived Si–B–C–N powders. Powders are synthesized in nitrogen or ammonia atmosphere to modify their carbon content. The nitrogen-treated powders sintered in the temperature range 1500–1900 °C lead to monoliths with a density increasing from 2.4 to 2.6 g/cm3, Vickers harness and Young's modulus ranging from 0.7 to 5.4 GPa and 48 to 102 GPa, respectively. SiC was the first phase to nucleate around 1700 °C, whereas Si3N4 grew at 1800 °C. At this temperature, monoliths are pure electrical conductor nanocomposites mainly composed of Si3N4 and SiC nanophases embedded in a turbostratic BN(C) matrix. Above 1800 °C, Si3N4 was not identified. In comparison, ammonia-treated powders sintered at 1800 °C represent dense insulator composites mainly composed of Si3N4 and BN with a Vickers hardness as high as 15 GPa and an excellent stability in air.