1. Remind people that this is just a game, and then hold them to good behavior.
2. Good behavior is not to fly off the handle when people play the game as they are supposed to and it just goes against you. People are supposed to get rid of the queen. People are supposed to try to get rid of their hearts.
3. People have to play the cards they are given. When they pass you the queen or a moon happens, practicing good behavior would say this is not your opportunity to "instruct" all the other players on how to play and call them every insulting name you can think of. Oh, and yes - sometimes non-bot players even make mistakes 'cause their human. Again, see above on good behavior.
4. There are no rules in hearts that you have to "protect" the high man or always hit the low man or swallow the queen to stop a moon even if it means (particularly if it means) sacrificing yourself. So, if other players don't play as you expect them too, again, don't take it as your opportunity then to throw tantrums.
5. Stop revenge playing in the room. Some players feel so entitled that they freely admit that they are trying to "punish" other players who had the audacity to pass them a queen or hit them when they are "high." Players who are subjected to this are unlikely to want to come back because they might be in a room with you again, so what have you accomplished?
6. Stop all the trash talk in the lobby. Jokes are one thing, but some players just can't keep their "feelings" about losing under control and then decide it's a great idea to start an argument for all to see. If someone does that in the lobby, they should be muted immediately and the person they accused should be allowed 1 opportunity to defend themselves. Only 1.
7. Don't ban players. There have to be other ways of enforcing rules and I've seen too many players (and has happened to me once) get unjustly accused by players who are favored and then banned with no way to defend themselves. Getting banned, particularly, when you can't defend yourself is a sure way to lose a player either for good or at the least for a long time. If someone is being abusive, have them forfeit a game or muted. But make sure it's applied consistently. And, banning people definitely will make people not feel good about the community and there are just too many other options if you want to keep this community alive. One way to control bad behavior is for everyone to speak out when a player engages in it. I also often see players say something insulting about another player or start trying to "punish" them in a game and other players not only don't speak up to say stop it, but they sometimes start to pile on the victim. If 3 players in a game told 1 badly behaving player to STOP and then IGNORE them, they'd soon stop since this behavior is mostly attention-seeking.
8. Most importantly, CALM DOWN if a TD makes a mistake. They are human too and volunteers, so while it might suck if you get missed somehow or a tourney gets screwed up (which is really rare), practice good behavior and patience with them too.
Likely I'll get grief over posting this, but if more people followed all this just plain courtesy you'd see many players want to come back and maybe even sign up as a host. Because I don't think people really want the tourney to disappear, but very few people want to continue to support, let alone play in, what they see as a toxic environment.
Hopefully food for thought.