Here is my idea for a easy anti afk system in Champion Select.
Give players the ability to vote that someone is afk, if they receive more than 2 votes, then popup a warning on the player who is afk. Allowing them to cancel the warning and prove they are at the computer.
This warning will have a button the player needs to press. 60 seconds if they have not yet picked a champion/afked into one.
After the last Character in Draft mode has chosen a champion then no one can vote kick anyone within a set time, to ensure no one is kicked right as they afk to wait for the game to start after actually selecting their champions.
In Blind pick it would allow it anytime, up until the last 10-20 seconds, if the player talks in chat then it won't matter if they afk at the end.
If the afk suspected player hits the button they cannot be voted afk again in the champion select.
Another mechanism that could be put into place to ensure people are not falsely voted against is make it so that if the player chats in the champion select menu, they cannot have a vote against them initiated.
Is it flawless no... def not, but less annoying than making players dodge.
In regards to ranked i would say give the afk the -elo hit, not a massive one but enough to discourage it.
TLDR - Vote afk in champion select with anti abuse mechanisms. Chat = can't be voted against.