  We develop a practical methodology to remove modes from a galaxy survey power
spectrum that are associated with systematic errors. We apply this to the BOSS
CMASS sample, to see if it removes the excess power previously observed beyond
the best-fit $\Lambda$CDM model on very large scales. We consider several
possible sources of data contamination, and check whether they affect the
number of targets that can be observed and the power spectrum measurements. We
describe a general framework for how such knowledge can be transformed into
template fields. Mode subtraction can then be used to remove these systematic
contaminants at least as well as applying corrective weighting to the observed
galaxies, but benefits from giving an unbiased power. Even after applying
templates for all known systematics, we find a large-scale power excess, but
this is reduced compared with that observed using the weights provided by the
BOSS team. This excess is at much larger scales than the BAO scale and does not
affect the main results of BOSS. However, it will be important for the
measurement of a scale-dependent bias due to primordial non-Gaussianity. The
excess is beyond that allowed by any simple model of non-Gaussianity matching
Planck data, and is not matched in other surveys. We show that this power
excess can further be reduced but is still present using "phenomenological"
templates, designed to consider further potentially unknown sources of
systematic contamination. As all discrepant angular modes can be removed using
"phenomenological" templates, the potentially remaining contaminant acts
radially.
