  Two-dimensional semiconductor moir\'e materials have emerged as a highly
controllable platform to simulate and explore quantum condensed matter.
Compared to real solids, electrons in semiconductor moir\'e materials are less
strongly attracted to the moir\'e lattice sites, making the nonlocal
contributions to the magnetic interactions as important as the Anderson
super-exchange. It provides a unique platform to study the effects of competing
magnetic interactions. Here, we report the observation of strongly frustrated
magnetic interactions in a Wigner-Mott insulating state at 2/3 filling of the
moir\'e lattice in angle-aligned WSe2/WS2 heterobilayers. Magneto-optical
measurements show that the net exchange interaction is antiferromagnetic for
filling factors below 1 with a strong suppression at 2/3 filling. The
suppression is lifted upon screening of the long-range Coulomb interactions and
melting of the Wigner-Mott insulator by a nearby metallic gate. The results can
be qualitatively captured by a honeycomb-lattice spin model with an
antiferromagnetic nearest-neighbor coupling and a ferromagnetic second-neighbor
coupling. Our study establishes semiconductor moir\'e materials as a model
system for the lattice-spin physics and frustrated magnetism.
