The unpleasant S sound (or any Sibilant sound) generally resides in the 5 to 8k area. You may be able to cut some in the EQ in the area and help your issue. A more advanced solution is a multiband compressor. Localize the frequency, center a band on it, and reduce the threshold. There are de-esser plugins in LPX, not really sure about GB as I have never really used it. But if the de-esser plugin exists in GB you could play around with that. I personally prefer the multiband comp approach though.