When someone abandons your game in Rainbow 6 Siege it is very simple. But before I explain it, if you happen to win the game with fewer players than the enemy team congratulations to you. And if you alone carried the game with missing teammates great job. But if you did not, oh damn.
When your teammate go full ♥♥♥♥♥♥♥ ♥♥♥♥♥♥ and leave your game then you should quickly boot the ♥♥♥♥ out of the enemy team and screech from the top of your lungs so your grandma ♥♥♥♥♥♥♥ dies of the shockwave. There is nothing you can do about it other than carry it. Ubisoft does not give a single flying ♥♥♥♥ about it so just deal with it mentally and enjoy your wasted 15-20 minutes.

This is way less common than the previous one and there is something you can do about this. But it requires you to have
1. Something to record with
2. The will to know your effort is wasted.
After the game do not worry, just head over to Ubisoft's website and report it and Ubi will respectfully return your lost MMR and nullify the match as if it never happened. But guess what this does not ♥♥♥♥♥♥♥ work even with a recording of the entire match and screenshots of bandit in the climb animation (used for teleporting in Siege) active with Zofia just behind him not seeing it because she just spawned which is physically impossible without teleporting. So enjoy your missing MMR, its a loss dude, live with it.

Do you have around 500-1200 games played in siege? Nice, you are probably between gold and plat somewhere. But wait, is that a ♥♥♥♥♥ with 12.000 games on the enemy team? WHAT THE ♥♥♥♥ UBISOFT? You think to yourself. DO NOT worry my friend I'm here to help you and share my ideas on what you should do. Just kill him lol, if he has 12k games played and end up playing with people with your amount he is guaranteed to be ♥♥♥♥♥♥♥ trash just slay his ass and call him out in the chat. However, if the case is that he is a dia- No wait Diamonds value their K/D too much to smurf, it is not gonna happen pal don't worry. But MAKE SURE TO CALL IT OUT IN THE CHAT THAT HE IS SO TRASH.

Thank you for bearing with and reading this ♥♥♥♥. Now for real if you see any cheaters, report to Ubi and hope they return your MMR and ban the ♥♥♥♥♥♥, it does not always happen even with solid proof because of the extreme laziness at Ubisoft, Sorry. Just keep playing and don't worry too much about it. People who accidentally or unintentionally leave the game are worried as hell and are likely deeply sorry on the inside and they will do what they can to return to the game. But those who do so on purpose auch, ♥♥♥♥♥♥♥ dox them and murder them for all I care. The community thanks you for it.