If you thought this (a picture) was annoying, wait til' you see this unsurpressable mess:
How do I stop command blocks outputting to the console?
Also, the following command does not work:
/gamerule commandBlockOutput false
It only suppresses the messages sent to the chat, as demonstrated by the screenshot in the link.
Therefore, it is not an acceptable answer. Unless you can prove me wrong.
The logAdminCommands gamerule will prevent logging commands and outputting to the console. The console will no longer show such logged messages (indicated by the "INFO" section), but the console will still show messages from chat (indicated by the "CHAT" section), which the commandBlockOutput gamerule controls instead.
/gamerule logAdminCommands false
As well, the sendCommandFeedback gamerule will suppress messages sent to the player's chat if they run commands themselves:
/gamerule sendCommandFeedback false
