In the console command input try using
Log [LogCategory] off
In your case:
Log LogViewport off
You can also update DefaultEngine.ini or Engine.ini:
LogViewport=all off
I haven’t tried the DefaultEngine.ini/Engine.ini approach, though. But the console command “Log LogViewport off” works. Perhaps not as nice as what you are suggesting.
Here is more info:
There is a handy free plugin on the market place that I use to help me keep track of useful console commands: