Here's an automateable idea for the dev team: Amulet-based performance floors, but only for top-rated players. Ban for 24 hours if top-rated players don't meet the floor.
Let's say I'm the top-rated player on my team, i.e. highest MMR rating. If I'm playing Berserker amulet, but I get 20% or less of my team's damage and kills... did I really try my best? I'm the best player on my team, I should be relatively outperforming my teammates. In my humble opinion, if you're the best player on your team and you're using Mender's amulet (even on sword Weaver), you should get at least 30% of team healing. If you don't, your rating is improper or you're throwing. So here's some ideas based on popular amulets, the percentages are just general ideas:
Sage: 25%+ kills and damage, 25% healing.
Carrion: 28%+ kills and damage.
Berserker: 28%+ kills and damage.
Demolisher/Marauder: 25%+ kills and damage.
Mender's: 30%+ healing, 28% defense.
To be clear, this should only apply to the top-rated player on each team. This is to penalize players deliberately throwing games, or for being generally unfit to play (drunk/need to sleep/very distracted). I don't think increasing MMR penalty is a good idea, it wouldn't really do all that much to really good players since they'll just rise back up quickly. But bans prevent them from throwing frequently and from annoying people with their unexpectedly bad play.