We perform first principles molecular dynamics simulations to investigate the magnetoelectronic response of substitutionally-doped boronitrene to thermal excitation. We show that the local geometry, size and edge-termination of the substitutional complexes of boron, carbon or nitrogen determine the thermodynamic stability of the monolayer. We find that hexagonal boron or triangular carbon clusters induce finite magnetic moments with 100% spinpolarized Fermi-level electrons in boronitrene. In such carbon substitutions, the spontaneous magnetic moment increases with the size of the embedded carbon cluster, and results in halfmetallic ferromagnetism above 750 K with a corresponding Curie point of 1250 K, above which the magnetization density vanishes. We predict an ultra-high temperature half-metallic ferromagnetic phase in impurity-free boronitrene, when any three nearest neighbour nitrogen atoms are substituted with boron, with unquenched magnetic moment up to its melting point.