I was very careful to specify "respond with the use of force as permitted by the UN Charter and international laws and norms." In other words the UN Charter only permits a response in proportion to the offense. I do think an act of cyber warfare may legally allow us of "armed force" but it would likely have to be limited to targeting the installations where the attacks were coming from (but realistically it is a new and undeveloped area of law with respect to cyber warfare).
The problem in my opinion with failing to act is we signal that there will be no military response, and these acts of cyber warfare escalate to hacking power grids or other infrastructure than results in indirect lose of life. Then due to political pressure all out war becomes more realistic.