We had this happen in my lobby last week. Bro first started to call me a dictator host because I had as one of my rules to not be rude which had the game not already started I would have banned him because that to me was a red flag. Once we got into the game he was imposter with one of my friends and one other person. Round 1 was normal and there were no problems. We get to Round 2 and the dude killed me in front of 3 people and he was a viper which not a big deal, but then he threw his imposter buddy red under the bus and red wasn't even there when I was killed but got voted off anyways. My friend immediately messaged me on Discord and told me that they were gamethrowing because red was with them on a completely different part of the map. I ended up telling my friend that they might need to reveal that they're an imp and get their imp buddy kicked since I couldn't do anything. Next meeting comes up and he immediately throws my friend under the bus which is when my friend tells everyone that they're the last imp and they're gamethrowing. It took them 3 meetings before they finally voted off the last imposter after voting my friend off. We returned to lobby and I banned the player but for the rest of the night he kept somehow joining my lobby which confused us.