What is the source, singing or speaking?
Type of mic can make a difference.
As Tobi said an EQ cut in the SSSSSS area can help. Start out with a wider
EQ bandwidth and then narrow it down as you dial the frequency into the problem area.
Sennheiser wireless mics (the analog ones) maximize their S/N by pre-emphasis/de-emphasis. This can easily result in overdriven high frequencies (even when you are far away from clipping) that you perceive as over-emphasised sibilance. Try to lower the gain in the transmitter.
And then the remaining sibilance can be removed with an EQ, as described above.