One AFK offense should not permanently ban someone's account. However, just like any frowned upon behavior, multiple offenses will have consequences. We're still working to improve the system(s), but any behavior of the sort should be reported via the tool.
If you encounter people going AFK in your games, please report them via the feedback tool in-game through the main menu (there's a section for "Report player").
We're working on improving and implementing more tools to combat toxicity in our game.