Be careful of certain bugs that cause persistent crashes. The forums often have posts labeled “Known Issue—crashes caused by certain reasons.” Players might experience consecutive crashes—for example, when Hive Scum was released, there was a bug where one player disconnecting or leaving the game would cause other players in the team to also disconnect, and that disconnecting player would continue to cause other players to drop out. Another example is when an event mode has a bug that kicks players as soon as they enter a mission, like the Rotten Armor event.
Another thread suggesting a punishment for leaving a team that can’t stay alive longer than 30 seconds and is doing between the three of them as much damage as you are yourself.
funny, the last 20 or so leavers i got on tape were havoc40s or high level players that tried to show off but fell short of their ability to do so 
quite the comedic relief come to think of it, i should keep a betting pool “who’ll quit first” 
In all of the games I’ve played the “punishment” method never works. If you get pissed enough to rage quit you quit. I doesn’t matter if you get a 15 min penalty or what ever it is.
This doesn’t help the team a single percent and is absolutely useless.
Imo what you need is rapid replacement.
Let the game open up a slot for someone new.
There are of course issues with this since there is the overlap with “someone left - maybe they will reconnect” and the time the remaining team is left hanging.
Technically you could keylog it, but then you could mod that out…etc. etc. If someone is willing to “cheat” one way, they’ll cheat the system in another as well.
It is a compromise, and there is likely always a bad part, even if we have a 5:th flex spot.