As in imaging one divides by the flat field.
For data taken between May 12th and July 24th 2001, the odd-even column effect is strongly present. It is difficult to remove from the spectroscopic flat fields, so the observatory has retaken flats for all spectroscopic setups that were taken during these dates. These flats are available from the ESO archive.
Observations with the medium resolution grating show fringing at the one percent level. If the flat was not taken immediately after the observations and if the telluric standard was not observed at the same place on the array, then fringing can be difficult to remove.