Why would you want to do that?
If you really want it, you can change it inside the services/zwave/config folder.
There is a zwave config file, allowing you to set the log level.
Or you can via ssh make the log file read-only:
sudo chmod 444 /volume1/web/PulseStation/Services/zwave/zwave.log
The option to set logging to no, only reduces it a little bit.