Hi everyone
I'm noting this down here because I haven't found much documentation about this on the web and discovered it while testing stuff.
If you want to completely disable logging, there should be a configuration file under C:\Program Files\NSClient++\NSC.ini ou C:\Program Files\NSClient++\nsclient.ini. Edit that file and comment the line FileLogger.dll. This line is in the [Modules] section.
Restart the nsclient++ service via services.msc and look at the size of nsclient.log: it won't increase or be created anymore.
NOTE:This configuration option should work with version prior to 0.4.0. More recent versions have the log file = none option in the configuration file to stop logging.