For Plesk version 10 & 11
====================
1. Log into Plesk control panel.
2. Select "Tools & Settings" from "Server Management" .
3. Select "Server Settings" from "General Settings".
4. Uncheck the box "log files and statistic reports".
5. Click on "OK" button.
For Plesk version 9
===============
1. Log into Plesk control panel.
2. Select "Settings" from "Main Menu".
3. Select "Server Settings" from "General" section.
4. Uncheck the box "log files and statistic reports".
5. Click on "OK" button.