  We present a new technique of decontaminating Swift UVOT grism spectra for
transient objects. We describe the template image requirements and image
processing steps necessary to successfully implement the empirical
decontamination technique. We demonstrate the accuracy of the flux and
wavelength calibrations for decontaminated spectra by comparing a spectrum of
SN 2011fe with a well-calibrated, long-slit ultraviolet spectrum from the
Hubble Space Telescope's Space Telescope Imaging Spectrograph. We also show how
the decontamination removes spurious emission lines from spectra of iPTF14bdn
which otherwise could be misinterpreted as coming from the supernova. The
software which implements this technique is briefly discussed and is made
available to the community.
