Here is a simple solution to AFK Farming.

Implement a system that triggers anytime when a player makes no kills, does no damage, or makes no meaningful movement between spawns. Once the system is triggered, upon the next death, display a message to the player with a random key/button press to confirm. For example:

Are you still here? Press <Random Key/Button> to continue. Press any other key to quit.

Add a 20 second countdown, then kick them if they do not press a button. This way, if they do nothing, they get kicked, or if they have a macro for button presses, it will be unlikely for them to get it right more than once out of chance, especially if the Character confirm key/button is always different than the AFK Confirm dialog.

Also, make it so none of the modes allow a player to enter the game without selecting a character, and any player with the AFK trigger dialog at the end of game, gets no credits... unless they press the correct button. In modes like HvV, allow the game to launch with 8 players, even if someone hasn't selected a Character, but do not allow the player to spawn in game without having selected a character. Add a 20 second countdown, then kick them if they do not choose a character.

With the inability to earn credits and without the ability to meaningfully interfere with others' games, AFK should stop almost completely.

Being that I am not a game developer, my use of the word 'Simple' should not be construed as to mean 'Easy'.