Tell them to stop doing that. If that doesn't work, don't heal them. If it still doesn't work out, quit and find a new queue. You can still even be civil before leaving. Just gauge where your responsibility ends and theirs begins.
Find a good guild to run with and this won't be a problem. Otherwise, it's just how things are. Playing a healer means dealing with people, but in most cases you can't control them any more than you can stop the boss from teleporting to do its special move.