There's a long history of how to deal with bad actors online. One that is somewhat rare, but effective is "disemvoweling", where the vowels are removed from the post leaving it... basically gibberish, but readable if you work at it.

There's other tactics like this, but the fundamental theory is that if you can connect someone's bad actions to them for everyone to know, for them to feel shame about, it can avert a good majority of bad behavior.

So my suggestion is to place a tag on a person's name when they've done something, such as left a draft, or went AFK, stuff that is checked automatically.

For example, everyone would see "[Draft Dodger]" or "[Draft Dodger x2]" appended to their name, during draft, in IM, chat, and in the game, so even the opposing team sees it. The tag would go away after a game or two games or whatever.

It becomes a mark of shame, that creates a "cost" that if you have a genuine reason, you can bear. For example, you lost connection. It's a bearable cost without being onerous on people who just have something shitty happen to them.

Developers and designers, as one myself, tend to try to create technical solutions for social problems. We need to be better at identifying social problems, and applying social solutions to them.