Firewall security can be set to Block All Mode either from the tray icon or via firewall behaviour settings. However, it will remain at the setting until manually changed and there is no way to set it to block all after a set period of network inactivity.
Just out of curiosity: why would you want this to be available?
Because of the ambiguities of “network activity”, you would probably only do it manually anyway. What you really want is to “pause” the network after a period when there is no “interesting network activity”. And defining “interesting network activity” is the crux of the issue. Networks tend to have a fair bit of traffic in spite of you. For example, without a router, someone will hit you with a TCP request every few seconds. So maybe restrict to “unblocked activity”? But if you have a router in the loop, there is the constant router to NIC chatter to deal with. So maybe “unblocked activity to the WAN”? But if you are really on a LAN, you will go down in spite of traffic on it. But hey, ■■■■■ them! So now you only need to contend with the websites that renew themselves, like yahoo.com, and figure out whether you count only new tcp connections, add in udp, count persisent connections that renew themselves, … . And at the same time keep from cutting off people in the middle of a big ftp or http download or ? The algorithmic and setup complexity can certainly exceed the utility and create a lot of traffic for the forum. And, of course, you may also need to deal with some of the blocked stuff when you manually bring the network back up. An automatic “pause” seems much more complex than a manual “block the network”. But maybe someone from Comodo has a solution and will comment on it.