If you look in the QVDistributionService install folder (by default in C:\Program Files\QlikTech, I believe), there should be a file called QVDistributionService.exe.config. Within that file, there's a line like:
<add key="Trace" value="false" />
It's near the bottom, under a line labelled *** Extended Logging ***. Can you confirm that the value is set to false?