DOES SONAR ARTIST HAVE AN DE ESSER??

There isn't a specific plug in included in Artist to do this.  However, I think you have access to the style dials in the Mix Strip.  You might try to use the Smoother one.

If there's nothing in the box, maybe try one of these. I don't know any to recommend.
http://www.best-free-vst....ffects/de-esser-01.php

There are plenty of free or inexpensive de-essers available. But if you want to stick to bundled plugins, then the Sonitus multi-band compressor can do the job, as suggested by Lord Tim.
 
Enable just band 4, set it between 4 KHz and 9 KHz and give it an initial compression ratio of about 8:1. These are starting points that you'll have to tweak based on the material. Set the attack time fast (1-3ms) and the release time medium (200-300 ms). Give it a sharp knee, maybe 3 or 4 in the "Knee" box.
 
Play back the track and watch the bouncing ball in the graphical display, using that as your guide for setting the threshold such that the bouncing ball only pops into the compression range during obvious sibilance but stays below the knee the rest of the time. Once you've found a threshold setting that works, raise the Ratio - you'll know when you've gone too far when the vocalist starts sounding like he's got a lisp.
 
Finally, adjust the bandwidth of the filter. Set the lower frequency as high as possible without impinging on the de-essing effect. You might find that the essiness doesn't start until as high as 7 KHz. The upper limit isn't as critical.