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Subtraction of astrophysical foreground contamination from "dirty" sky mapsproduced by simulated measurements of the Murchison Widefield Array (MWA) hasbeen performed by fitting a 3rd-order polynomial along the spectral dimensionof each pixel in the data cubes. The simulations are the first to include theunavoidable instrumental effects of the frequency-dependent primary antennabeams and synthesized array beams. They recover the one-dimensionalspherically-binned input redshifted 21 cm power spectrum to withinapproximately 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 theEoR signal. We find that the weighting function used to produce the dirty skymaps from the gridded visibility measurements is important to the success ofthe technique. Uniform weighting of the visibility measurements produces thebest results, whereas natural weighting significantly worsens the foregroundsubtraction by coupling structure in the density of the visibility measurementsto spectral structure in the dirty sky map data cube. The extremely denseuv-coverage of the MWA was found to be advantageous for this technique andproduced very good results on scales corresponding to |u| < 500 wavelengths inthe uv-plane without any selective editing of the uv-coverage.

Foreground Contamination in Interferometric Measurements of the Redshifted 21 cm Power Spectrum