  The construction of catalogues of a particular type of galaxy can be
complicated by interlopers contaminating the sample. In spectroscopic galaxy
surveys this can be due to the misclassification of an emission line; for
example in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) low
redshift [OII] emitters may make up a few percent of the observed Ly${\alpha}$
emitter (LAE) sample. The presence of contaminants affects the measured
correlation functions and power spectra. Previous attempts to deal with this
using the cross-correlation function have assumed sources at a fixed redshift,
or not modelled evolution within the adopted redshift bins. However, in
spectroscopic surveys like HETDEX, where the contamination fraction is likely
to be redshift dependent, the observed clustering of misclassified sources will
appear to evolve strongly due to projection effects, even if their true
clustering does not. We present a practical method for accounting for the
presence of contaminants with redshift-dependent contamination fractions and
projected clustering. We show using mock catalogues that our method, unlike
existing approaches, yields unbiased clustering measurements from the upcoming
HETDEX survey in scenarios with redshift-dependent contamination fractions
within the redshift bins used. We show our method returns auto-correlation
functions with systematic biases much smaller than the statistical noise for
samples with at least as high as 7 per cent contamination. We also present and
test a method for fitting for the redshift-dependent interloper fraction using
the LAE-[OII] galaxy cross-correlation function, which gives less biased
results than assuming a single interloper fraction for the whole sample.
