  We present a theory of a single-electron transistor exchange-coupled to a
localized spin. We show how to gain detailed quantitative knowledge about the
attached spin such as spin size, exchange coupling strength, Land\'e g-factor,
and spin decay time $T_1$ by utilizing a robust blockade phenomenon of DC
magnetotransport with accompanying noise enhancement. Our studies are of
particular relevance to spin-resolved scanning single-electron transistor
microscopy, electronic transport through nanomagnets, and the effect of
hyperfine interaction on transport electrons by surrounding nuclear spins.
