Overwatch "leavers" are players who, for one reason or another—maybe their internet went out, or maybe they got mad because everyone on their team sucks and it's all their fault and arrrrgh—bail out of a game before it's properly concluded, leaving their teammates at a significant disadvantage. Penalties that keep them from immediately joining new games are already imposed on those who do it, but game director Jeff Kaplan said in a recent forum post that even harsher sanctions, including possible permabans, are on the way.
"Our philosophy has been that we would rather not have leavers playing the game at all (especially in Competitive Play). We keep increasing the penalty for leaving and will continue to do so," Kaplan wrote. "We're in the process of implementing a new policy which would take into account how many Competitive Seasons you have been banned from and at a certain point, prevent you from playing Competitive ever again."
He also said that Blizzard has been working on ways to address the problem of players who intentionally throw games. "We had a lengthy meeting about this yesterday. We've been doing a lot of research on the problem for months now. We think we have some good solutions," he wrote in a separate post. "We still need players to help us by reporting people. We do both manual and auto detection on our end, but player reports are the most immediate indicator that point us in the right direction. We also discussed the need for more feedback for the person doing the reporting so it doesn't feel like your report is going into the void."
Blizzard recently suspended Overwatch pro Dafran from the first two seasons of Overwatch Contenders for behavior that included, among other things, intentionally throwing matches, something he admitted to publicly during a (mildly NSFW) Twitch livestream.