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: