If you don't have a DeEsser plugin handy (which is usually just a sort of single-band dynamic equalizer) You can do some straight-EQing to get rid of it by looping a section of sibiliance and sweeping a high-gain, High-Q EQ parameter around until you hear it really bring out the sibilance. DeEssing is really difficult to do if you're stuck with really harsh sibilance.
