It turns out that you can "instruct" syslogd so that it ignores all messages from Solar Service instead of writing them to /var/log/asl. Here's how:

1. Add the following two lines in /private/etc/asl.conf:

# Ignore messages from Logitech's Solar Service.app
? [CS= Sender Solar] ignore

2. Reboot

OR simply restart syslogd by running:

sudo launchctl unload /System/Library/LaunchDaemons/com.apple.syslogd.plist
sudo launchctl load /System/Library/LaunchDaemons/com.apple.syslogd.plist

Sources where I found all this information:
http://superuser.com/questions/278231/how-to-disable-set-logging-level-preferences-per-mac-os-x-appl...
http://robaldred.co.uk/2009/05/restarting-syslog-process-on-mac-osx

Source