Why would you want to do that ?
It is just reading in historical data from a file, C:\Program Files\Alwil Software\Avast4\DATA\log\nshield.log, the last 10 entries in that log. You could empty it using notepad but you would first have to disable the self-defence module or that would block you.
You could also disable Logging in the Network Shield, Customize, Settings, but I thing that would be a mistake.
This information that is stored and displayed could help us to help you if you experience a problem and you would be saving very little is space is the motivating factor.