  Recent atomic force microscopy (AFM) experiments~[ACS Nano {\bf 2014}, 8,
12410-12417] conducted on graphene-coated SiO$_2$ demonstrated that monolayer
graphene (G) can effectively screen dispersion van der Waals (vdW) interactions
deriving from the underlying substrate: despite the single-atom thickness of G,
the AFM tip was almost insensitive to SiO$_2$, and the tip-substrate attraction
was essentially determined only by G. This G vdW {\it opacity} has far reaching
implications, encompassing stabilization of multilayer heterostructures,
micromechanical phenomena or even heterogeneous catalysis. Yet, detailed
experimental control and high-end applications of this phenomenon await sound
physical understanding of the underlying physical mechanism. By quantum
many-body analysis and ab-initio Density Functional Theory, here we address
this challenge providing theoretical rationalization of the observed G vdW {\it
opacity} for weakly interacting substrates. The non-local density response and
ultra slow decay of the G vdW interaction ensure compensation between standard
attractive terms and many-body repulsive contributions, enabling vdW {\it
opacity} over a broad range of adsorption distances. vdW {\it opacity} appears
most efficient in the low frequency limit and extends beyond London dispersion
including electrostatic Debye forces. By virtue of combined
theoretical/experimental validation, G hence emerges as a promising ultrathin
{\it shield} for modulation and switching of vdW interactions at interfaces and
complex nanoscale devices.
