  We present first results from a galaxy population study in the highest
redshift galaxy clusters identified in the 2500 deg$^2$ South Pole Telescope
Sunyaev Zel'dovich effect (SPT-SZ) survey. The cluster selection is to first
order independent of galaxy properties, making the SPT-SZ sample particularly
well suited for cluster galaxy population studies. We carry out a 4-band
imaging campaign with the {\it Hubble} and {\it Spitzer} Space Telescopes of
the five $z\gtrsim 1.4$, S/N$_{SZE}>$5 clusters, that are among the rarest most
massive clusters known at this redshift. All five show clear overdensities of
red galaxies whose colors agree with the initial cluster redshift estimates.
The highest redshift cluster in this sample, SPT-CLJ0459-4947 at $z\sim1.72$,
is the most distant $M_{500}>10^{14}~M_{\odot}$ ICM-selected cluster discovered
thus far, and is one of only three known clusters in this mass range at
$z\gtrsim 1.7$, regardless of selection. Based on UVJ-like photometric
classification of quiescent and star-forming galaxies, the passive fraction in
the cluster central regions ($r/r_{500}<0.7$) is higher than in the field at
the same redshift, with corresponding environmental quenching efficiencies
typically in the range $\sim0.5-0.8$ for stellar masses
$\log(M/M_{\odot})>10.85$. We have explored the impact of emission from star
formation on the selection of this sample, concluding that all five clusters
studied here would still have been detected with S/N$_{SZE}>$5, even if they
had the same passive fraction as measured in the field. Our results thus point
towards an efficient suppression of star formation in massive galaxies in the
central regions of the most massive clusters, occurring already earlier than
$z\sim1.5$. [Abridged]
