  Subtraction of astrophysical foreground contamination from "dirty" sky maps
produced by simulated measurements of the Murchison Widefield Array (MWA) has
been performed by fitting a 3rd-order polynomial along the spectral dimension
of each pixel in the data cubes. The simulations are the first to include the
unavoidable instrumental effects of the frequency-dependent primary antenna
beams and synthesized array beams. They recover the one-dimensional
spherically-binned input redshifted 21 cm power spectrum to within
approximately 1% over the scales probed most sensitively by the MWA (0.01 < k <
1 Mpc^-1) and demonstrate that realistic instrumental effects will not mask the
EoR signal. We find that the weighting function used to produce the dirty sky
maps from the gridded visibility measurements is important to the success of
the technique. Uniform weighting of the visibility measurements produces the
best results, whereas natural weighting significantly worsens the foreground
subtraction by coupling structure in the density of the visibility measurements
to spectral structure in the dirty sky map data cube. The extremely dense
uv-coverage of the MWA was found to be advantageous for this technique and
produced very good results on scales corresponding to |u| < 500 wavelengths in
the uv-plane without any selective editing of the uv-coverage.
