To all chat in-game, you either type /all before your text, or hold Shift+Enter to immediately type in all-chat.
I don't know why this topic keeps popping up, seeing as there's another just like it.
If you feel like you don't want to hear someone being an idiot over all-chat, ignore him. You can do this via /ignore <playername> I think. If you type /ignore enemy, you ignore ALL members of the enemy team. If you type /mute, you ignore everyone, including your own team (I find this incredibly stupid, because this results in no communication at all except for pings, but that's your choice).