  We experimentally demonstrate that the decoherence of a spin by a spin bath
can be completely eliminated by fully polarizing the spin bath. We use electron
paramagnetic resonance at 240 gigahertz and 8 Tesla to study the spin coherence
time $T_2$ of nitrogen-vacancy centers and nitrogen impurities in diamond from
room temperature down to 1.3 K. A sharp increase of $T_2$ is observed below the
Zeeman energy (11.5 K). The data are well described by a suppression of the
flip-flop induced spin bath fluctuations due to thermal spin polarization.
$T_2$ saturates at $\sim 250 \mu s$ below 2 K, where the spin bath polarization
is 99.4 %.
