The simple fix to leavers is to have a sliding scale of lost mmr the higher you climb. So if you are at 2400 mmr and you realise your mmr is gonna drop to 2385, and you drop game resulting in a forfeited match, your mmr drops to 2300. On top of the 100 point mmr drop you should receive a 30 minute deserter debuff, that compounds each time you leave up to an hour, with a weekly reset. Drop out of more than 5 matches in a week, be put on a week long deserter debuff for ALL queued content, skirmishes, bgs, lfr, lfg, EVERYTHING.
Note* The deserter week long deserter debuff would be just that, a 7 day debuff, not one that resets with realms.
Throw the book at these people who waste everyones time.