Select region to process.
Effects > Amplitude > Dynamics Processing.
There are a few de-esser presets.
Lower threshold = easier to set compressor going (perhaps unmusically?)
Higher ratio = more agressive de-essing (ie compression in the sibilant frequency range)
Adobe Audition 3 is a Beast. That sh*t makes the cloverfield monster look like a treasure troll, lol. Check it out on adobe's website. If I didn't know SONAR like the back of my had I would've scooped this up the day it was released.