This I'm having a hard time understanding.
It's the host who owns the game, he or she should be the one who decides whether or not they'd want someone in their game.
But, maybe you could add a good/bad rating at game end, letting players rate the host, maybe even with specific reasons after the rating, which should be a unique vote, meaning only 1 per person so it wouldn't be abused. And then when a player is hosting a game, the rating could appear with the map/game name, as well as the most rated reason. That way people would hopefully try to avoid being a bad host.
I hope it made sense, hard to write on my phone.